You can upgrade from the Free version to the paid Premium versions inside the Weebly Site Builder itself:
While logged in to Weebly, click on the down arrow next to "MacHighway" on the upper-left:
This will take you to your MacHighway Client area where you can select the plan you want:
You can select any billing cycle you want, regardless of what cycle your hosting plan is on. Click on "Choose Product", where you'll be taken to a screen to enter your billing information. When payment is complete, you'll be logged back in to Weebly and you can use your new features.
There is an extensive knowledgebase with tutorials and information on every feature here: http://help.editmysite.com/
If you need additional help you can create a support ticket, or if you need immediate assistance please give us a call 1-877-288-3273 (US/Canada) or 1-540-737-3549 (International).