Hi,
First make sure you used the mojoportal download that says "formono" in the name.
Second, I'm not sure the Mono packages for Debian are new enough, do you know what version of Mono?
A better place to post issues about mojoportal on Mono is our Linux/Mono forums. I'm not a Linux guru myself but others who monitor that forum may have experience with running mojoportal on Debian. I use OpenSuse.
From what you've said it does not sound like mono is handling the request. If you are creating an apache virtual directory named "mojoportal-test", make sure that points to the actuall folder wwwroot which is meant to be the root of a mojoportal app. It can either be a root level web site at the host name or ip level or 1 folder deep as a virtual directory.
Best,
Joe