Are you using mojoportal source code or release packages?
If you have the mojoportal source code and you have MonoDevelop, you can open up the mojoportal.mds in MonoDevelop and find out if it builds.
If you are using pre-compiled release files, you should be using the "mojoportal-2-2-7-7-pgsqlreleasefiles-for-mono" package, then get the replacement data layer dlls from the mysqldatalayer download and put them in the /bin folder overwriting the pgsql version.
Hope it helps,
Joe