Thank you, Paul.
The new code:
Public myRibbon As IRibbonUI
Sub OnRibbonLoad(ribbon As IRibbonUI)
Dim oRibbon As IRibbonUI
Set oRibbon = ribbon
oRibbon.ActivateTab ("MyTab")
End Sub
This works perfect on my machine. I tried to install the same add-in on a different machine and there the ribbon appears and all other macros work well - but the ribbon is not in front, when opening PowerPoint. I remember, one reason could be the PowerPoint version, but as mine is 2010 and the other one is 2013 it shouldn't be the point. Or am I going wrong here? Are any other possible reasons known?