Does anybody know how to unlock weapons like the concret donkey or the Sentry gun for multiplayer. Somebody told me that i had to finish the story mode completly with beating all the times, managing all the deathmatches and chalenges and gettin all the trophys. can anybody confirm this. :-)

You have to get everything perfect. All easter eggs, time bonuses, costumes, weapons, schemes, trophies, and then...
A new scheme will be in the item shop....2500 credits to buy....THE KITCHEN SINK!
It has every single weapon infinite and no delay, including the secret weapons. But no, you can't make custom amounts of those weapons, only base schemes off of it.

Does anybody know how to unlock weapons like the concret donkey or the Sentry gun for multiplayer. Somebody told me that i had to finish the story mode completly with beating all the times, managing all the deathmatches and chalenges and gettin all the trophys. can anybody confirm this. :-)
As said above this post, you can get the Concrete Donkey in a scheme. Note that in having it, the scheme will also have every other weapon unlimited too.

The sentry gun is really the only superweapon that you can have in a scheme, but keeping it balanced. When creating a new scheme, base it off the Darksider scheme. (you may have to buy a few weapons in the shop to be able to get it) This will put 3 sentry guns with no delay into your scheme. (although each team is not allowed to have two sentry guns on the map at the same time)