In my experience it is more preferable to set your PATH environment variables appropriately instead of trying to locate executables on a per-app basis.
You can set your path so that the desired Ruby takes precedence over any other rubies and I am sure that RubyMine will pick it up (or just use ruby and let your PATH specify which to use).
On another note you might also want to have a look at a ruby version manager such as rbenv or RVM.
Update: Just tried to set manually a Ruby SDK in the latest version of RubyMine (6.3) and it worked as expected. I had, however, to expand the folder in which the binary resided (in your case bin
), select the actual binary and then click OK. Have you tried that as well?