Amazon's Kindle for Android application uses an internal serial number that's 72 character long. Extracting that serial number is a little tricky, but worth it, as it then allows the DRM to be removed from any Kindle ebooks that have been downloaded to that Android device.
Please note that it is not currently known whether the same applies to the Kindle application on the Kindle Fire and Fire HD.
Obtain and install adb (Android Debug Bridge) on your computer. Details of how to do this are beyond the scope of this help file, but there are plenty of on-line guides.
Enable developer mode on your Android device. Again, look for an on-line guide for your device.
Once you have adb installed and your device in developer mode, connect your device to your computer with a USB cable and then open up a command line (Terminal on Mac OS X and cmd.exe on Windows) and enter "adb backup com.amazon.kindle" (without the quotation marks!) and press return. A file "backup.ab" should be created in your home directory.
At the bottom-left of the plugin’s customization dialog, you will see a button labeled "Import Existing Keyfiles". Use this button to import the ‘backup.ab’ file you obtained by using the adb command. The backup file will be processed to extract any serial numbers in it, and the numbers will be added to the list.
On the right-hand side of the plugin’s customization dialog, you will see a button with an icon that looks like a green plus sign (+). Clicking this button will open a new dialog for entering a new Kindle for Android serial number.
Click the OK button to save the serial number. Or Cancel if you didn’t want to enter a serial number.
On the right-hand side of the plugin’s customization dialog, you will see a button with an icon that looks like a red "X". Clicking this button will delete the highlighted Kindle serial number from the list. You will be prompted once to be sure that’s what you truly mean to do. Once gone, it’s permanently gone.
Once done creating/deleting serial numbers, click Close to exit the customization dialogue. Your changes wil only be saved permanently when you click OK in the main configuration dialog.