Use B5 4BU in your sat nav for the Bullring Centre Car Park.
From the north: leave the M6 at junction 10, then take the A454 (signposted Walsall), followed by the A4148 (signposted Ring Road, Cannock, then Ring Road Birmingham) and then the A34 into central Birmingham (signposted City Centre and then Bullring, Shopping). Keep following the signs to Bullring and park in the Bullring Centre Car Park.
The route is extremely well signposted but if you want detailed directions from your starting point use Google's or the AA's free route planners. Type in the postcode B5 4BU for Bullring. The entrance is on Small Brook Queensway.
From the south: M1 or M40 to the M42 (junction 4) then follow the A34, signposted City Centre, then Bullring. Park in the Bullring Centre Car Park (follow the instructions above under 'north').