Yes, this is a recent issue where the devices.txt files does not contain the right entries. See this post of mine for how the devices.txt used to be.. All you need is to add the following line to your devices.txt file in the XLoader directory using any text editor:
Mega(ATMEGA2560);atmega2560;stk500v2;115200;
I thinking that the V1 documentation should include a link to a “correct” devices.txt file for XLoader since the download no longer contains an entry for the right board.