Missions made using Gunslingers editor addon (meaning, made in OFP v1.46 and below)
CANNOT be played in resistance! This is due to BIS changing the requirements for addon declaration in res. So like Armstrong said, you need to play the mission using v1.46, using Gunslinger's editor update in your addons folder (NOT the res/addons folder!)
Insert your CD, and click on the 'preferences' button on the auto-run program. At the top, there should be a combo box that says "Product:", and "Operation Flashpoint Resistance" is selected. Click on this box, and change it to "Operation Flashpoint", then click start. Now you should be running v1.46.
Remeber to replace Gunslinger's update with Kegety's when you are through with the mission though, since Gunslinger's isn't compatible with res. Also, remember to switch back to using Resistance in the preferences (reverse of what you did above).
Have fun