If you look in the /default/includes/ folder, you'll see a file called addonused.lpr.
In that (open with notepad or similar), you'll see a line
#include( "./driftmeter.lpr");
Just remove the # symbol (lapper ignores everything on line after a hash), save and reboot lapper
Do a few laps, then type !drf to see drift score in a table.
If you don't see any score, then you will need to amend your LFSLapper.lpr file.
In your LFSLapper.lpr file, search for $DefaultTopCar.
You probably have something like $DefaultTopCar = "XFG"; which is probably not the car you're drifting with.
Instead, make it either $DefaultTopCar = "XRT"; (if this is car you drift with, or change car name to the one you drift with), or, to make sure you record scores for every car, then change to
$DefaultTopCar = "UFR+XFR+RB4+RAC+FZR+XRR+FXR+FBM+LX4+LX6+XFG+XRG+XRT+FZ5+FXO+FO8+FOX+MRT+UF1+BF1";