You'll need a free account at Phonegap Build to automatically build your apps and test them on your device, visit build.phonegap.com to create your account.
Connect Reactor to Phonegap Build
Login to your Phonegap Build account. Click on the user icon at the top right and go to 'Edit Account', then click the 'Client Applications' tab. Copy the Authentication Token. Login to your Reactor account, go to Apps->Phonegap Account, add the token to the appropriate field, and save.
With that Authentication token in place, you can automatically build your app in Reactor. After building, you will see a QR code that you can scan with your phone to install the app right away.
Note: you do not need to use the "Register App" portion of the Phonegap Build interface. If you do not see an Authentication Token, you will need to create one, or reset/delete the current one until you see a token.
Testing Apps On Your Device
You can scan the QR code with any free barcode scanner app to get the app onto your device. Android requires going to the Downloads folder, and clicking the downloaded item to install it.
iOS apps will not work without going through the certification process first, see this article for instructions. Android apps will work right away without any certification.
Note: free accounts on Phonegap Build only allow you to build 1 app at a time. If you see any errors building apps, it may be because you hit your limit. You will need a paid Phonegap Build account to build more than one app at a time.