How do you add an interface for garageband ?

You probably need to supply more details with model name etc. Did it come with software and instructions? Have you visited the manufacturers website for details?

As a rule of thumb, there's three general steps I go through when adding an interface to GB.

First, make sure you have the right driver on your computer. You probably got a copy of the driver when you bought the thing, but better yet is to download a fresh version from Tascam's website. Drivers are frequently updated, so if you download it from their website, you'd be getting the latest greatest.

Second, physically connect your interface to your computer. Usually this is with a USB or Firewire cable.

Third, select your interface in GB preferences.