Let me preface this with the fact that if you're running one of the already supported OSs (debian or unbuntu for instance) you shouldn't follow this guide - instead you should just look on the mono website for information.

First thing I did was grab the most up-to-date mono and xst sources (i matched versions)

cd /tmp
wget http://ftp.novell.com/pub/mono/sources/mono/mono-2.6.tar.bz2
wget http://ftp.novell.com/pub/mono/sources/mod_mono/mod_mono-2.6.tar.bz2
wget http://ftp.novell.com/pub/mono/sources/xsp/xsp-2.6.tar.bz2

Then unpack everything

tar -jxvf mono-2.6.tar.bz2
 tar -jxvf xsp-2.6.tar.bz2
 tar -jxvf mod_mono-2.6.tar.bz2

then build mono first (change anything on the ./configure line to your liking, I do suggest picking a place to put mono, letting it install wherever it wants to makes the rest of the process very painful!):

cd /tmp/mono-2.6
./configure --prefix=/opt/mono
make ; make install
echo export PKG_CONFIG_PATH=/opt/mono/lib/pkgconfig:$PKG_CONFIG_PATH>>~/.bash_profile
echo export PATH=/opt/mono/bin:$PATH>>~/.bash_profile
source ~/.bash_profile

after a bit you'll have mono installed

Now build xsp (i had to add pkgconfig to an environment variable to get it to work)

cd /tmp/xsp-2.6
./configure --prefix=/opt/mono
make ; make install

Finally build mod_mono, you'll probably need to tell it where apache's apr-config is... it took a while for me to find it.. heh

find / -iname apr*config
cd /tmp/mod_mono-2.6
./configure --prefix=/opt/mono --with-mono-prefix=/opt/mono --with-apr-config=/usr/bin/apr-1-config
make ; make install

you should now find mod_mono in your apache modules dir (in my case /usr/lib64/httpd/modules/)

ls /usr/lib64/httpd/modules/mod_mono*
17 Feb  5 22:54 /usr/lib64/httpd/modules/mod_mono.so -> mod_mono.so.0.0.0
145511 Feb  5 22:54 /usr/lib64/httpd/modules/mod_mono.so.0.0.0

the mod_mono install seems to have automatically created mod_mono.conf in my httpd/conf folder, but you may want to make sure it's there

vim /etc/httpd/conf/mod_mono.conf

# mod_mono.conf

# Achtung! This file may be overwritten
# Use 'include mod_mono.conf' from other configuration file
# to load mod_mono module.

<IfModule !mod_mono.c>
    LoadModule mono_module /usr/lib64/httpd/modules/mod_mono.so
</IfModule>

<IfModule mod_headers.c>
    Header set X-Powered-By "Mono"
</IfModule>

AddType application/x-asp-net .aspx
AddType application/x-asp-net .asmx
AddType application/x-asp-net .ashx
AddType application/x-asp-net .asax
AddType application/x-asp-net .ascx
AddType application/x-asp-net .soap
AddType application/x-asp-net .rem
AddType application/x-asp-net .axd
AddType application/x-asp-net .cs
AddType application/x-asp-net .vb
AddType application/x-asp-net .master
AddType application/x-asp-net .sitemap
AddType application/x-asp-net .resources
AddType application/x-asp-net .skin
AddType application/x-asp-net .browser
AddType application/x-asp-net .webinfo
AddType application/x-asp-net .resx
AddType application/x-asp-net .licx
AddType application/x-asp-net .csproj
AddType application/x-asp-net .vbproj
AddType application/x-asp-net .config
AddType application/x-asp-net .Config
AddType application/x-asp-net .dll
DirectoryIndex index.aspx
DirectoryIndex Default.aspx
DirectoryIndex default.aspx

Now, you need to get this to work with ISPConfig3, since if you've followed my other tutorials, that's what you have running things on the back end.

Modify your httpd.conf file to include mod_mono.conf. You want to do this at the very bottom, just before the NameVirtualHost directives (ISPConfig puts 3 directives at the very bottom of httpd.conf, you want to place this line just before them:

vim /etc/httpd/conf/httpd.conf

Include /etc/httpd/conf/mod_mono.conf
# this is where ISPConfig's stuff is, don't modify it, this comment is so you know where to put the line above the comment 
NameVirtualHost *:80
NameVirtualHost *:443
Include /etc/httpd/conf/sites-enabled/

now, restart apache

service httpd restart

and open up ISPConfig. go to sites > website > Select the website you want to add a mono enabled directory to, then click on the "Options" tab for that site. Enter the following into the Apache Directives (replace /var/www/path/to/the/web/folder with the path to that site's web folder)

MonoPath default "/opt/mono/lib/mono/2.0"
MonoServerPath default /opt/mono/bin/mod-mono-server
AddMonoApplications default "/test:/var/www/path/to/the/web/folder/test"
<location /test>
	MonoSetServerAlias default
	SetHandler mono
</location>

and finally restart httpd

service httpd restart

here's a test page, create it in your /var/www/path/to/the/web/folder/test directory and set the permissions to whatever the other site's permissions are, or apache.apache

<html>
<body>
<% Response.Write("Hello World!"); %>
</body>
</html>

now, once ISPConfig restarts apache (sometimes this is instant but i've seen it take up to 5 minutes) you'll see Hello World! displayed