[horde] Login Config help

[Jeff]

I installed the latest HEAD versions of the framework, Horde, Imp/Dimp, 
kronolith, turba, etc.
Everything is configured with a MySQL backend and it works great.
 
When users login, they first login to horde, then have to login again to 
mail. I want them to go straight to Dimp. Apologies if the answer is 
obvious, but I've read the wiki
several times and looked everywhere in horde/config and I just don't get 
it. (Just tired?) This has to be simple but I don't see it.  Could 
someone point me to a
HOWTO or just tell me what options to edit?