I just had this exact same problem. Clips from my FCP 7 sequence would not show up when I sent them to Color 1.5. No way. No how.
I solved my problem (at least for now) by simply restarting my machine. I realized that it had been running for days, and it probably just needed a reboot. Try it. It might work.
And yes, I'm running Mountain Lion, too. 10.8.2.
Good luck! ----Todd Ruel