1. Confirm that you have a SketchBook Membership in your iTunes Account.
2. Download SketchBook from the Mac App Store on your Desktop and run it.
3. Once SketchBook runs, you should be able to login using your email using SketchBook from a mobile device.
4. Buy SketchBook Membership again, if possible.
5. You may need to contact Apple, if the membership doesn't start up when you launch SketchBook.
Video demo: http://autode.sk/1PBBzp3
NOTE: You can also Google: contact itunes support and it will find the closest location for Apple Support. i.e. EMEA, ASIA