Re: The requested name is valid, but no data...
Each data layer uses its own connection string and doesn't know about the connection strings for the other data layers so there is no need to remove anything. You do need to make sure the connection string for MySQL is correct but the reason its trying to use postgresql is because the version of mojoPortal.Data.dll in your bin folder is the one for postgresql.
If you want to use MySQL, you need to download the MySQL Data layer from the download page and replace the mojoPortal.Data.dll in the bin folder with the version for MySQL. You also need the other dlls in the bin, MySql.Data.dll and sharpziplib.dll
When working with the source code in VS, you just change the project reference to use whichever data layer you like. The above is the instructions for release files which is what I think you are using.
Hope it helps,