Hi Kathy,
Searching on the Plan -> User Stories page isn't too easy. I'd recommend you try either the Custom List app (you can add this app to your dashboard), or the Plan -> Work Views page, where you can create a view for your specific project/s and specify the Work Item Type being 'User Story'.
In either way (Custom List or Work Views) you shall then see if you can find your custom field, then add it to your filters and search this way.
Please let us know if that helped. If not, then perhaps include a screen shot of how you're trying it and we'll take it from there.
Thanks,
Sagi