The Usa agario skin is available in game! To play Usa simply name yourself "Usa" in the agar.io game menu. Your cell's skins will then automatically show up. This functionality works on the agar.io main server, however for some skins you may need to download a mod in order to play it. There are also multiple add-ons you can download on Chrome via the 'add extensions' page, which will allow you to play users skins on the agario main server.
We've provided a visual example below to explain exactly what to do. Please use this as a guide, and remember to type in the skin name EXACTLY how it appears. If done correctly, you should be able to play the skin in game. Keep in mind however that only official skins will be available using this feature. If you'd like to play a user skin you'll need to use a mod or play on a private server.
We have over 10,000 user uploaded skins on AgarioSkins.com! Many of the user skins are currently not available in game yet, however the developers may consider adding the most popular user uploads to the official game later on. Browse through our user uploads and rate your favorite agar.io skins! You can also create your own skins and submit them to us for review. We receive hundreds of user skin ideas every day! Many of them are absolutely amazing ideas. We encourage you to give it a shot and try to create something interesting. It's a lot of fun and can possibly make it into the game.
If you'd like to discuss Agario and share skin ideas with hundreds of other players, we've set up an awesome forum site which now has a great community. At AgarioForums.com you can discuss anything to do with the game, find cool mods, watch crazy videos, seek Agario help and watch tutorials, and stay up to date with the latest news. If you're an Agario fan like myself, this site is the perfect community to share your experiences.