Hi Rogan,
If you are working with the source code you don't need any additional downloads, though I recommend you get the code from svn trunk instead of using the .zip.
All you need to do is change the reference from the mojoPortal.Business project to the data layer of your choice, in this case from mojoPortal.Data.MSSQL to mojoPortal.Data.MySql. Then rebuild the solution.
Note that the code from svn is more up to date than the release .zip In recent days I've implemented the MySql data layer for the survey feature and will soon do the same for the Poll feature. To use the Survey you would either use mojoportal-complete.sln or mojoportal-survey.sln, and just as with the main business layer, you need to change the SurveyFeature.Business so it references SurveyFeature.Data.MySql instead of SurveyFeature.Data.MSSQL
Survey and Poll are implemented as separate projects to illustrate how developers can create features that plug in to mojoportal without mingling their custom code in the core. They use post build events to copy their output to the proper location beneath the Web project.
If you are working with the release files (which are pre-compiled and don't include any C# source code) you would follow the instructions here. I've updated it to reflect that sharpziplib is no longer needed.
Hope it helps,
Joe