Per Ridex in an email to me:
"In Admin. go to Categories / Products until the item is listed...
Look to the description at the right (where the wee thumbnail is)
Press 'Stock' - for T-shirts on demand, just put in large notional stock
It's not obvious but the Add button should read Add/Change - please make a request in bugtracker to remind us to do this."
I'm following his orders, LOL. This is a reminder!