It will lock in. The very front of the detach. mag is a notch, this needs to find the lip inside your SKS magwell, it is actually part of the barrel lug, I think. Anyway, this is what also acts as a notch for your "permanant" mag. Put the detent in the notch and move the magazine upwards into your rifle. It will then snap into place, the "mag release" will now allow your entire magazine to be removed, as opposed to simply opening the trapdoor.
Be advised: SKS milling and the workmanship of these 30 rounders are inconsistent at best. You are very likely to have to work parts of the mag and perhaps even the rifle i/o to make this fit. Then, you will have a 30 rounder that is likely to perform only slightly better than a USA mag in an AR. When kept in original config., the SKS is a GREAT tool, why mess with it?