help digitize vinyl with GarageBand...

I'm trying to convert my vinyl records to digital with GarageBand. I have done this in the past with a desktop Mac, using an RCA-to-1/8"-headphone-plug converter. The new MacBook Pro doesn't have 1/8" audio in, so I got a Technical Pro USB2RCA converter, and it works fine but with one major problem: it's not stereo. Is there some way to change this?

When I create a new track and select Input 1+2, there are two channels but the right one gets no signal. If I select just Input 1 or Input 2 (each says USB AUDIO CODEC) there is only one channel. It's irrelevant, by the way, which USB port I use.

The old 1/8" headphone jack worked perfectly on the Mac, so I'm wondering if the problem is with my new USB2RCA converter, or if it's a simple fix in the settings either in the MacBook or in the app.

Is anyone having success doing this sort of thing and/or able to point me in the right direction?

Thanks!