Driving from Frankfurt:

Get on A66 going toward Wiesbaden.
Take exit 4-Wiesbaden-Biebrich toward Wiesbaden/WI-Biebrich
Keep left at the fork, follow signs for Wiesbaden-Biebrich
Turn left onto Biebricher Allee/K643
Continue to follow K643 (get into the right lane)
Turn right onto Äppelallee/L3482 (Traffic light)  (see Note! below...)
Turn right onto Gibber Straße (Traffic light)
Take the 1st left onto Seligmann-Baer-Platz
Seligmann-Baer-Platz  (This is the parking lot for the Bahnhof)

Note! There likely won't be much parking.  I STRONGLY suggest you continue on Äppelallee one more block, and look for the sign indicating Parking at the Penny grocery  Store.  Take a right into the parking lot, continue past the Adler to the back of the lot, and follow the drive left.  You will see a parking ramp ahead of you about one block or so.  There is no fee to park there, and the walk back to the train station will take you only about 2 minutes.

From central Wiesbaden:
find your way to Biebricher Allee and follow directions above.

From Mainz:
take A643 toward Wiesbaden/Mainz-Gonsenheim   
Take exit 3-Wiesbaden-Äppelallee toward Wiesbaden-Schierstein/Wiesbaden-Biebrich   
Keep right at the fork, follow signs for WI-Biebrich/Gewerbegebiet/Rheinfeld and merge onto Äppelallee/L3482   
Turn left onto Gibber Straße  (if parking is needed, see above.  The Parking ramp will be on your left, so you may need to turn earlier and find your way to the back, behind the stores.)

Alternate from Mainz:
Cross the bridge at Mainz-Kastel, and follow Wiesbadener Strasse to your left.  This will turn into Äppelallee at the light at Biebricher Allee.  See additional directions above.