For satellite navigation reference, the postcode for Alton Towers Resort is ST10 4DB. Please set to your sat-nav to stick to ‘main routes’ or ‘fastest’, not ‘shortest’ which may end up taking you no further than a farm!
If you don't have sat-nav you basically need to get off the M6 at J15 or J16 and take the A500 to the A50 towards Uttoxeter, Alton Towers is on this road.
From the M1 get off at J28 and follow the A50 towards Stoke.
Alton Towers is very well signposted as soon as you get anywhere near so you really shouldn't have any problems finding it.
There is ample parking on site, be prepared for queues at peak times though.