i've been playing really really hard and incredibly hard level for quite some time. however the stupendously hard level seem to never 'open up'. is it because i'm not suscribed or do i have to wait longer
The upper difficulty levels will open up to you once you reach a certain number of experience points. For stupendously hard, i believe you have to have 10,000 experience points.
If you keep your mouse on an unavailable difficulty level, you will eventually see a liitle side note that says (for example) min. exp. 10,000.