Choose "Create New XDF Parameter" on the XDF menu.
Choose if its a scalar or table.

You will then get a window where you can input the data about the map.
The "General" tab is info about the map, the address, the data type and unit.
The "Rows" and "Columns" is about the units, addresses and data types for the map axis.
The "Conversions" lets you define a formula for translating the map values to a unit that is more human readable.

To get a hint how it works right click on a map in the Parameter Tree and chose "Edit Parameter XDF Info"

hi the definition file is for my car. ive done really well finding the maps and labeling them to my best but im  stuck ive got loads of maps i dont know what they do and also i cant find the maf maps.
http://rapidshare.com/share/9478E4347A13C29EA03DF5308AF02131

I would be gratefull if anyone could help.

You can find a similar ECUs xdf and just copy the xdf parameters out of it. Paste them into your new xdf and change the memory addresses to the values you have found.