From the East - King of Prussia & Philadelphia, New Jersey:

New Jersey:  PA Turnpike West to Exit 326 (Valley Forge).  Go through toll plaza and take first exit on right (Valley Forge).  Follow to Route 23 (approx 4 lights).  
Then see BOTH

Philadelphia:  Schuylkill Expressway West to Valley Forge Exit 327 Mall Blvd.  Turn Right on Mall Blvd.  Go to stop light, turn Right on to Gulph Road.  Follow to Route 23 (approx. 5 lights).
Then see BOTH

BOTH:  Route 23 West through Valley Forge Park and Phoenixville.  Left on Route 113 South (McDonalds on Right).  Follow approximately 1.5 miles, Fairgrounds are on the Right.

From the West - Harrisburg & Reading:

Harrisburg:  PA Turnpike East to Exit 312 (Downingtown).  Route 100 South to Route 113 North.  Cross Route 401.  Continue to traffic light (5 miles) at Coldstreams Road.  
Continue on Route 113, Fairgrounds entrance 1/4 mile on Left.

Reading:  Route 422 East to Route 724 (past Pottstown).  Turn Left off exit ramp onto Route 724.  Follow Route 724 till it ends and becomes Route 23.  Follow Route 23 East 
to Route 113 South (McDonalds on Left).  Turn Right onto Route 113, follow approximately 1.5 miles, Fairgrounds are on the Right.

From the South - Delaware:

I-95 North to Route 202 North.  Continue to exit for Route 401.  Turn Left off of exit ramp onto Route 401 West.  Continue to Route 113 North.  Turn Right onto Route 113.  Follow to Fairground entrance on Left, 1/4 mile past light at Coldstreams Road.

From the North - Allentown:

Northeast Extension South to PA Turnpike Exit 326 (Valley Forge). Go through toll plaza and take first exit on right (Valley Forge).  Follow to Route 23 (approx 4 lights). Route 23
West through Valley Forge Park and Phoenixville.  Left on Route 113 South (McDonalds on Right).  Follow approximately 1.5 miles, Fairgrounds are on the Right.