Open coin door, look to right side or along the top edge, look for any handle or anything the pulls or moves to unlock the lock down bar.

LTG : )™

Open up the front coin door.
Reach in and slide the lockdown bar lever near the top righthand inside the game towards the left.
Remove lockdown bar and slide glass off carefully.

After you open the coin door you should see a bar on the right side of the opened coin door hanging down.Pull that bar to the left to release the lockdown bar.
lockdownbar 001.jpg
lockdownbar 002.jpg

Instead of a lockdown bar, they may have just used two small bolts. Check to see if there is a small bolt and nut hanging down from the lockbar. A lot of older Gottlieb's used this method. After you get the lockbar off and the playfield glass removed, you may discover that the playfield is actually screwed from the top down. It won't lift up until you remove the 6-8 screws holding it down. Good luck.

Post a pic of what your game has (lockdown bar, inside the coin door, etc.)