If you are traveling by your own car you may park outside the YHA. 

Get to the south side of Tower Bridge 
and head down Jamaica Road (the A200) 
You'll come to the Rotherhithe Tunnel roundabout. 
Take the exit off the roundabout 
BEFORE the Tunnel into Brunel Road. 
This becomes Salter Road SE16. 
You'll pass Rotherhithe Station on your left. 
You'll go over a bridge and see the YHA on your left. 
Turn left after it and left again - 
you'll see the Spice Island Pub on your right.  Park. 

Buses that run to the YHA: 381, N381, 225, 395. 

Tube: Rotherhithe (East London Line) 300 yards