i think it is recommended to change the charcoal canister every 30k miles. my c220 has 95k miles on it and i have not changed it yet. anyway, have you looked inside the left-hand rear wheelarch liner yet? it has an inlet and an outlet pipe and an air inlet cap and it's mounted by a couple of bolts only. should be straight forward. hope this helps.
basically, the canister collect the fuel vapor from the fuel tank and then recycles it. the charcoal canister is only purged when the engine is under load so it won't supply an over rich mixture during idle.