Let go step by step.
Go to Extention manager, when it opens,
go to click Browse to locate the extention you downloaded
Click Uploade & Install to install it.
If it extension you installed, go to Module manager, look for the extention name, tick it and at the top click publish or If the extention(depends on the what the extension does) has some features to add eg picture, special effect, click on it to add the necessary things, after that check Publish (under status) and click save to have start working.