You have to disassemble the SKS to remove the magazine.  This is pretty simple; there's a button you push at the rear of the trigger guard to disconnect the trigger group from the receiver.  At that point, you can remove the fixed magazine and reassemble.  Removable mags attach after reassembly.

Honestly, there is no good way to mount an optic to an SKS.  But it seems that everyone has to learn this for themselves, which is why every SKS owner buys one or two (or three or four) of the different mounts to try to get things working.  Eventually, most folks revert back to irons, and toss the mounts.  Also, the scopes that come with many of these packages are super crappy and will break and fog after a few outings.  You just won't get quality for $20.

I recommend getting the Williams rear peep sight for the SKS.  For most folks, it will be a far better solution than a junk scope on a junk mount.

-Troy