Bushy Run Battlefield is located just east of Harrison City on route 993.  One of the best approaches to it is from Business route 66.  For example, from the intersection of routes 66 and 22 at Delmont, take route 66 south for 0.9 miles to exit 12 (the sign will say Business 66 and Last exit before toll road).  At the light, turn left and take Business 66 south 1.7 miles to route 993.  Turn right and go 3 miles.  The entrance to Bushy Run Battlefield will be on the right.

Alternatively, from route 30 in Greensburg, take Business 66 north and turn left onto route 993 and go 3 miles.  The entrance to Bushy Run Battlefield will be on the right.

Also, for those coming from Pittsburgh on route 22, you may turn right on Harrison City Rd (at the Choice gas station).  Go for 4.5 miles to the traffic light in downtown Harrison City (where route 993 goes through town; it is a light at which you must turn).  Turn left onto 993 and go 1.4 miles.  The entrance to Bushy Run Battlefield will be on the left.
 
Here is a Google maps link to the parking area at Bushy Run Battlefield.

The GPS coordinates of the parking area are 40.358103 N, 79.626180 W