My stock release button was a little stiff as well. It actually got stiffer the more I messed around with it. I believe that's because these parts have the same finish - inside and out - as the rest of the rifle, which is that rough parkerizing. Here's what I did:
First, open and close the folding stock about 20 times. Be sure that you're not putting and pressure on the stock itself when you press the button, as torquing the stock can cause the mechanism to bind. I find it's easiest to flip the rifle around and grab the back of the receiver for leverage while I depress the button with my thumb, but YMMV.
Next, disassemble the folding stock mechanism, which is extremely simple. The button on the bottom of the mechanism is also the retaining nut for it. Unscrew the button counterclockwise by hand. The button will have a spring that nests in it. Remove both the button and the spring from the bottom, and you can slide the locking pin/bolt out the top of the mechanism. You can now remove the buttstock section from the mounting plate.
Now, clean and lube the inside of the mechanism, as well as the inside of the button, and the outside of the locking bolt. You'll likely have a decent amount of black "dust" inside the mechanism, which appears to be from the finish on the parts wearing in. Clean all of that out of there, and be sure the mechanism is well oiled/greased.
Finally, reassemble the mechanism (the reverse of the disassembly). Align the folding stock piece (either extended or folded), and insert the retaining bolt. Flip the rifle over, and put the spring in place on the bottom, over the threaded bottom portion of the retaining bolt. Then install the release button by placing it over the spring, compressing the spring slightly, and tightening the button/nut clockwise.
Once you do this, the release button should be much easier to operate.