In case that doesn't work please make sure the game is running and the affected game is loaded and open the psychoff/data/encounters directory. This is in the following locations (assuming you chose the default installation directory)
- Windows: C:\FrozenSynapse\psychoff\data\encounters\
- Windows, Steam: C:\Program Files\Steam\steamapps\common\frozen synapse\psychoff\data\encounters\
C:\Program Files (x86)\Steam\steamapps\common\frozen synapse\common\frozen synapse\psychoff\data\encounters\
- OS X: ~/Library/Application Support/FrozenSynapse/psychoff/data/encounters/
- Linux: ~/.local/share/FrozenSynapse/psychoff/data/encounters/
This should resolve the problem. Please reply if it doesn't, or if you need any help finding this file.