Wep Hack MK1S ASAP NEED HELP

To all who reads this...... Hi.

im asking for help to those who know how to equip the MK1S Red from the Weapon Hack
i could see it in my Inventory but i cant equip it...
It doesn't say if its for Slot 3 or 5 it is just Equip/Delete button
When i tried to equip it says Not available for branch or whatever that was...
please help are there any things i need to do before i equip it??
i also have a 14th day 5th slot Please Help ASAP

Im using this wep hack of http://www.mpgh.net/forum/showthread.php?t=798299

Credits to: People who are involve
to the Answerer...
to the thankers.....

if any of you got offended by ASAP im sorry ^_^ me just desperate for MK1S T_T