As with bitcoin, the value of Carboncoin depends on two things:
1) The number of people who use it
2) the number of payments made using it over a certain amount of time
This means that in order to help Carboncoin to plant trees all you have to do to help with 1) is get yourself a wallet with some coins in it and tell your friends to do it too and 2) accept Carboncoin as payment for your business if you have one and let anyone you know who has a business know that they can do that too.
The real beauty of the project is that it rewards everyone who holds coins as well as planting trees, because if we all get 1) and 2) to happen, then the value of all of the coins goes up the same amount at the same time.