it may not be possible to fit it into that compartment, if yours has the removable internal divider, you may have to remove it and then I recommend trying to stuff the bag into the bottom. 

Many folks line the interior of the pack with something like a large trash bag for waterproofing first and then stuff the bag into the bottom. 

Stuffing fills up all the space, which a compression bag may not do.

good luck!

REI Member Since 1979 YouTube.com/philreedshikes