I downloaded the zip file but i don't know what to press to activate the account.
What do you mean? No activating/registering is needed to run MyBB. Its free and Chris has said it always will be.
If you are wondering how to unzip the file you downloaded, usually you right click on the folder and choose "Extract All" and follow the instructions.
I think this person thinks it is like proboards, and YOU run it for them.