That's weird...Never had that happen before. You are a Full System Admin? Are there anything in the logs when you system is starting up? Also, did you grab the Admin Essentials version from the latest 4.0 downloads on the Download page? I wont lie...I havent had anyone ask me about 4.0.x in a while...so will need to take a look.
Never had an issue though with the Tab not showing up....so something must be erroring out causing the Admin plugin to not register all together.
Let me know. =)