That error tells no lie, it can't connect to your MySql db.
Are you using MySql for the db? That doesn't work well in Medium Trust unless the host has the correct version of MySql connector installed in the GAC on the server.
I added the key to the web config
There is no need to add a key the connection sting setting already exists in the appSettings section of Web.config for all the dbs, you just have to pick the one for your db platform and set it to correct connection string value. ie. for MySql its this one you need to edit, ideally put it in user.config:
<add key="MySqlConnectionString" value="Data Source=localhost;Database=mojoportal;User ID=mojouser;Password=mojo123;Charset=utf8;" />
Hope it helps,
Joe