Hi Kiyoshi,
What version of Mono are you using? 1.2.4 or higher is needed.
The root of your web would need to be /var/www/mojoportal/ so it will correctly find the bin folder.
To rule out (or rule in) if it is a mod_mono configuration issue, you can try running the site from the command line with xsp2
cd into the /var/www/mojoportal/ and do
MONO_OPTIONS="--debug " xsp2
then you will be able to open a browser and navigate to http://localhost:8080 to see the site. If there are errors you may get more information from the console output. If it works then the problem is probably something about your mod_mono/apache configuration. I have some notes I made a while back about mod_mon/apache configuration here, hopefully they are still accurate.
Hope it helps,
Joe