I only have 1 name and it's already taken

Your IMDb account is not automatically linked up to an IMDb name page, although I believe that you can 'claim' a name page to your profile if you use IMDb Pro. If the profile name Cherry has already been used on IMDb you will have to choose something else. When you register the account you can add your full name, and in some places, like reviews, you can select whether you want your full name or your account name to be shown.

As far as adding yourself to the movie goes, you only need a verified IMDb account to do so. After registering an account you need to verify it by confirming your e-mail address. Once your account is verified, you can add and change information on movie pages, just like anyone else with a verified account. You can then go to the movie page in question and use the Edit page button at the bottom, to add your credits. If other actresses share your name you can select to add yourself as "Cherry (II)" or whatever number comes next to those already in the database. That name has nothing to do with the profile name on the IMDb page, so your movie credit will still show your real name even if your IMDb profile does not.

As well as everything cartman says, it might be wroth waiting a couple of weeks if the title has just been added - your credit may have been submitted and hasn't ye appeared yet.