Okay, taking a look now. Not seeing it.
On the Ground Units tab, in the section of "Transfer Ground Unit to PDC or Population" that's where I can select the original or one of my 2 single-battalion PDCs. I transferred one Garrison each into those. Now, I can select the PDCs by name, but the garrison units don't come up, so I can't see a way to transfer.
Ditto if I go to the tasks group section. The PDC-Mars fleet lists the two garrison battalions, but there are no actions I can take with that fleet.
I'm sure it's obvious but I just need the button, y'know? Button, button, who's got the button . . . . ?