How can Tor Browser Bundle for OS X access my keychain?
In short: You can't. Firefox always uses it's own store for passwords and will never access the Mac OS X keychain at all. You either have to manually copy your credentials into Torbrowser/Firefox windows or use a third party software like 1Password that integrates across browsers.

Be aware that if you use a login on a website that hasn't been created over tor there is a good chance you're giving up your anonymity by yourself. Tor cannot protect you from posting your identity (by logging in with a username and password) by yourself!