Sometimes dashlaunch may not let you select the file for XRPC.dll so instead copy your Launch.ini from the root of the usb, HDD or device in use to your pc and manually edit it with notepad. Just go down to the plugins and type in XRPC.dll in an empty slot. Save it and put it back into your device. Insert your device and reboot your xbox and launch dashlaunch. Once you're in dashlaunch check your plugins and it should be there.
If you still can't see it, go to the Save / Load launch.ini section and make sure you have loaded the correct launch.ini from your device. Simply press 'a' on the correct device and check your plugins, if it is there you can proceed to delete all other launch.ini or just copying your current settings to your other devices by using the 'x' button.