So you have something like this right?
Pull that white lever to the back, the side that doesnt have the words.
Then turn the scope around to the other side and you should see this
You can see in the picture that theres a silver knob thing at the front of the rail part. That thing is suppose to go between the 2 rails on the side of the rifle. So put that into the rails and it should just slide on.
If that doesn't work, you are probably trying to mount an AK scope on a SVD.