The README explains that a little. If you do not use FreePBX, the button.cfg definition for parking looks like this:

[PARK/default]
type=park
extension=700
label=Parking
context=parkedcalls

In order to park calls you have to add a context to asterisk:

[fop2-park]
exten => _X.,1,Set(ARRAY(RETURN_EXTENSION,RETURN_CONTEXT)=${CUT(EXTEN,:,1)}\,${CUT(EXTEN,:,2)})
exten => _X.,n,ParkAndAnnounce(PARKED,,Console/dsp,${RETURN_CONTEXT},${RETURN_EXTENSION},1)
exten => _X.,n,Goto(${RETURN_CONTEXT},${RETURN_EXTENSION},1)
exten => _X.,n,Hangup

If you use Asterisk 1.4.29.. DO NOT USE PARKING, it will segfault your PBX. Upgrade or Downgrade Asterisk to avoid problems.

How did you try to get the call back? Clicking on the parked call will show you a popup menu with the pickup option, you need the pickup permission in order to use that. You can also dial the parking extension or just let the call timeout and it will bound back to the parking extension.