If you find yourself running into a feature not enabled in the library you can always send it as a raw message to ANT using the ANT_Device.writeRawMessageToDevice(byte msgID, byte[] msgData) command.
For enabling only RSSI:
byte[] msgData = new byte[2] {0x00, 0x40};
writeRawMessageToDevice(0x6E, msgData);
Cheers