Hi Lars,
I found the problem, there is a bug. To solve it edit the file /Setup/applications/sts-SiteMembership/FeatureDefinitions/SiteMembershipModule.config
This line:
supportedDatabases="MSSQL,MySql"
needs to be changed to:
supportedDatabases="MSSQL,MySQL"
note the case of letters is the problem, it must be MySQL not MySql
Then visit the setup page again.
I will fix this and update our package.
Thanks,
Joe