ARTSTOR is part of JSTOR, so you can use your same personal account.
Do this first: Create an account in JSTOR. Visit the Using Databases-->JSTOR tab for a guide on how to set up an account.
Have a JSTOR personal account and password? Go to ARTSTOR
After clicking on "Link Existing Account" put in your JSTOR login email and password. Click "Link Account."
If you see your email at the top right of the page, you are logged in!