Monster Manual Drag tokens are blank

I purchased Odyssey of the Dragonlords.  It does not have permission to use D&D official artwork however, I also own the Monster Manual and when I try to replace one of the monsters from the MM library into my play area, it comes in blank (specifically I tried to move the 'mimic').  When I drag and dropped, it just showed a blank round token (it has the word mimic and has the correct stats).  Additionally, is there a way to use the mimic character sheet to roll to hit and damage or do I have to do that manually?

You need to make it so that Roll20 will not default to the version of the monster already in the Journal. IIRC, you can do this by deleting the monster in the Journal (can't be undone easily) or renaming it. Then you should be able to drag the monster onto a map and have it populate your Journal. You should be able to click on the name of attacks on the mimic character sheet and have a result show up in Chat. It's also fairly easy to create a button that will show up at the top left of your screen for attacks whenever you select a token, but I won't go into macros now.