You don't actually edit php files. In your admin there have been new options to enter your credentials into the form fields of your user profile.
You should never need to edit WordPress core files or plugin files to get a plugin to work. If it's not working there is something else going on with either your Nike+ credentials, you data coming from Nike+, or some conflict in another plugin or your theme.
It is also entirely possible there is something within the Nike+ plugin but as I've not had anyone else with this issue I have no way of knowing what that might be.