That means you are hosted in a Medium trust environment. mojoPortal can run in Medium trust but it requires some configuration changes in Web.config. If you look carefully in web.config you'll see comments about things that need to be changed for Medium trust.
Mainly, you have to uncomment Lucene.Net.lockdir and put the server file path to the index folder so the search engine can function, and you have to comment out the HttpUploadModule
More info here:
http://www.mojoportal.com/mediumtrust.aspx
Hope it helps,
Joe