Arrakis, I can imagine what you mean. The button sequence to get into some of the settings can be a bit confusing.
The computer will activate at around 3m of depth. You can put the computer into Dive Mode manually using the buttons. It's the mode after PC. Press left hand button once from PC. (Can't remember what it is the other way).
If it's a new computer these points won't be relevant but if it is second hand you'll need to check it's not set in Bottom Timer only mode. I can't remember of the top of my head which "Set" this is under. You also need to check it's configured for Air rather than Nitrox (assuming your just using Air).
If your familiar with the "Sim" mode you can actually simulate how the computer will behave under water. If you go to this option and hold down left button, the computer will show you it's "dive layout", you can then descend/ascend using the left/right buttons (it runs faster than real time). If you see a no deco minutes figures on the bottom left then it should be fine.
I'm not sure if you've got the manual but you can download it from this URL in PDF format.
http://www.mares.com/pdf/M1.pdf.
Finally the other things I've done are turn the beeps off. These were very annoying as it's very easy to activate the fast ascent warning when your in less than 10 metres of water. I look at my computer a lot so I'm confident that I won't miss anything by having beeps off. This includes warnings for no deco/batteries, etc so use with caution.
I've also turned the lockout function off. If you get a fast ascent warning (I think even once) during a dive the computer locks into bottom mode until the residual nitrogean counter returns to 0. This means no more diving with the computer that day. You can turn this "feature" off through the Set Mode. Again you need to be happy with doing this but there is a Rocket symbol that shows an uncontolled fast ascent. I certainly wouldn't recommend doing anymore diving for 24 hrs if you get one of those.
Give me a shout if you need anymore info. Sorry if I've gone on a bit
.