From the dashboard, choose a plan. Click the files tab to get to the file browser. Navigate to the file or folder you want to compress, then click the clipboard icon beside it to add it to your clipboard. Next, navigate to the folder where you want to put the compressed file or folder archive, and click the compress files link on the right under Clipboard items.
Note: Any items on the clipboard at the time will be compressed, so before you clipboard an item, make sure you empty the clipboard by clicking the link on the right under Clipboard items.