This usually happens because of various reasons.
- First, install the mod by copying the mod files into a folder in the Gamedata/ folder (For this, probably a "construction" folder).
- Than, delete all .cs.dso files from your gamedata/ folder and sub-folders (Please take note that some mods have a folder in them that says do_not_delete or something of that matter and the folder contains some .cs.dso files - dont delete those!).
- Launch the game using the shortcuts inside the mod. (If there isn't a shortcut, create one that points to tribes2.exe with the arguments -online -mod construction tacked on).