Assuming you donated with PayPal DURING the Kickstarter, you will receive all the same Kickstarter rewards that are listed for backers. The $30 Kickstarter level was First Flight. It included the following: Adventure Mode + KS exclusive Dust Rider 2.0 + CE Skirmish Mode (Skirmish Mode soundtrack, Captain's costume pack) + Dust Rider 1.0 costume
You should have already recieved an e-mail containing the Skirmish Mode, soundtrack, Dustrider and Captain's Costume Pack keys. You should also have recently got another e-mail with the code for the new Skyrider costume (Dustrider 2.0). Assuming that you already have Skirmish Mode, that key is yours to do with as you please! Roll up a secret level 1 player for yourself on a new Steam account, give it to a friend, try to leverage it in a trade (keep in mind that you will NOT be able to activate the code on Steam for an inventory gift/trade item, as is standard with ALL 3rd party Steam activation keys), or just save it for a rainy day.
To activate the Dustrider and Skyrider costumes, while in-game, click the "Store" button. From there, click "Redeem Promo Code" at the bottom, and enter in the codes from your e-mail. If you did not already have the soundtrack or Captain's Pack, you can activate those codes through Steam to add that content to the game you already have. The soundtrack is located in C:\Program Files (x86)\Steam\steamapps\common\Guns of Icarus Online\Guns of Icarus Online Soundtrack Alternately, you can right click Guns of Icarus in your Steam library, click properties, then the "Local Files" tab. From there, the "Browse Local Files..." button will take you to the folder containing the soundtrack. The Captain's Pack is 6 predetermined costumes, one for each class and gender. It will directly apply the new costume items to your inventory under "Role". If you already had any of this extra content, the codes are also yours to do with as you see fit.
When Adventure Mode is released next year, you will get an e-mail containing the activation code for the Adventure Mode DLC on Steam. If it ends up being necessary for Muse to release Adventure Mode as several smaller packages, you will receive a separate e-mail containing a key for each DLC as it is released. It will not matter how you acquired GoIO on Steam, be it retail purchase, Kickstarter, or Indie deal. As long as you already have Skirmish Mode you will be able to activate the key(s) for the Adventure Mode DLC.
The suggestion to donate $10 now is for anyone who missed the Adventure Mode Kickstarter, or only were able to back at the $5 level at the time. The donation is the best way to be sure to get the entire Adventure Mode experience as thriftily as possible, while still helping to support the development. If you donate the $10 now, you will not receive any of the bonuses I mentioned above. You will only receive the key(s) for the Adventure Mode DLC when they are released, and you MUST already own Skirmish Mode to active the keys.
Hope I answered all the questions for now!