Come to think of it, it's possible that Toast can only mount plain bins, not bin/cue pairs. I don't have any to test with at the moment. I've always mounted bins using Toast's "Disk Image" section, but it also has a separate option for Bin/Cue files, and I don't see a Mount button there. (Edit: just like ElDiablo said. We posted at the same time.)
Anyway, here's how to mount plain bins:
1. Click the Copy icon at the top of the Toast window.
2. In the drawer, click on the "Disk Image" button (if the drawer isn't visible, click the little blue arrow button at the left of the window).
3. Drag your bin file into the Toast window, and click the Mount button.