Hi David;
This has been an issue with PB for sometime. You can add a DB Function to any of your ASE databases - but, the DB Painter never displays them! I have noticed that and reported this in many PB beta tests. 
However, if your try and use the functions in your ASE database - they should work. 
Since IM is really PB in disguise, the DB Painter behaves the same (no DB functions displayed).
I would suggest opening a ticket on this with SAP Technical Support.
Regards ... Chris