The xml is the file that contains all of the settings you have.
There is one xml file for each profile you have, some people have multiple profiles.
To find the xml open Mach and look at the profile name, if you are just using the default plasma profile it will say Plasma lower right of the screen.
Next close Mach and go to the Mach3 folder on your hard drive and look for a file with the same name as the profile, it will have a .xml file extension, so if it is the default the file you are looking for is plasma.xml.
You will need to copy and paste that to your desktop and then rename it to something unique so that the forum will accept it, suggest you rename something like daveclegg.xml.
Once you have done that you can use the additional options button on the reply page to browse to your renamed file and attach to the forum.
Hood