NOTE: In the below, substitute a space for the underscore
to make the paths work!
In Windows 7 and Vista, another place to put Addons is "C:\Users\<username>\AppData\Longbow_Digital_Arts\Tread_Marks" (For XP, it's the same path except using "Documents_and_Settings" instead of "Users".)
(for example, if your username is "foobar", then put that in the area shown as "<username>".)
Does work...just place Addons directory in that path.
(example from my Windows box: "C:\Users\rdanner3\AppData\Longbow_Digital_Arts\Tread_Marks".)
Took a little thought on my part, because (since there could be a storm anytime here) I'm on Linux Mint 10, which gives me significantly better on-battery time.