Sorry I don't know much about Plesk.
I think possibly you do not have .NET 3.5 SP 1 installed. You may have 3.5 but without the needed SP.
Also if you have .NET 1.1 apps running on the same machine you must use a different application pool for those than the one you use for 2.0/3.5
Also you can easily install the latest mojoPortal using the same Platform Installer. The version in the Gallery at the moment is 188.8.131.52 but if you you open up IIS console, you will see near the bottom rigt "Import Application", using this you can browse to the mojoportal-2-3-1-5-mssql-deploymentfiles.zip or the very latest which is version 184.108.40.206, and it will install it the same as if it came from the Gallery.
Hope it helps,