On Adobe Stock, you need to use your Adobe ID and password to log in. If you don't have one yet, it's simple to create one. Your Adobe ID is your email address, and you can set the pw to whatever you want.
If your email address matches your Fotolia email address your account is synced automatically. If not, you are prompted to sync your account. You then need to enter your Fotolia user name and password - these are Fotolia specific. If you forgot your Fotolia pw, go to the Fotolia website and choose the "Forgot password" link. See niktol's instruction to click on the "return to Fotolia legacy site" if you see the Adobe Stock interface on Fotolia's website.