I assume that you have edited the AGKHelper.java file (and make sure that the string is properly closed).
Did you do a refresh of the project after you edited the file? (Select the project in the project explorer in Eclipse and then hit the F5 key, and there is no feedback when you do this.)
Are those the only errors/warnings showing up?
I just looked at your original post again, and should have paid attention to "(in src>com.thegamecreators.agk_player.until)".
I have no idea what "com.thegamecreators.agk_player.until" is. There is no directory or file named 'until' (if you replace the '.' with '\') under IDE\interpreter_android_prebuilt\src\thegamecreators\agk_player.
The error you indicate is probably showing up because something else isn't correct somewhere in your setup, not in the actual Base64DecoderException.java file.
I was just able to import the interpreter_android_prebuilt file into Android and only had to add the usual android-support-v4.jar file as an external jar (project properties->Java Build Path:Libraries tab) and it 'ran' fine.
Cheers,
Ancient Lady
AGK Community Tester and AppGameKit Master