Hi,
I would start with Basic Troubleshooting to find out the error details.
Did you install the mojoPortal package for .NET 4 or did you install the mojoPortal package for 3.5 .NET under .NET 4 hosting?
You should use the .NET 4 package of mojoPortal if your hosting is .NET 4.
If you are hosted in medium trust see the Medium Trust documentation.
Hope it helps,
Joe