mojoPortal targets .NET 3.5 SP1.
If you need a build for 2.0 .NET you can either use mojoportal 2.2.7.8 which was the last release package to target 2.0 .NET, or, it is still popssible to make your own 2.0 compatible build by using Visual Studio 2008 and changing the target back to 2.0 .NET yourself, then if you get any build errors just exclude any files it complains about. You can package your own build using the free UnleashIt tool as described here. I produced a build like this a few weeks ago for one of my customers so I know it can still be done.
Hope it helps,
Joe