When you are mounting a derailleur that already has the chain running through it you have to twist the derailleur back quite far to get the b-tension screw to catch its tab... much farther than the derailleur will sit once it is mounted.
Three things you should do before mounting a derailleur with the chain installed are remove the rear wheel, back out the B-tension screw, and take the chain off the chainrings in the front... this should minimize the amount you need to turn the derailleur and give you the slack you need.
Or you can break the chain and install the derailleur sans-chain, then reassemble the chain with a new quicklink.