It turns out that it is "missing" the folder "Assets" to be able to load it as a mod.
The simplest way is to create a mod with your preferred name and then create in it a folder called Assets.
Extract the rar wherever and place the two folders (Art and XML) directly in the Assets folder of the newly created mod.
That should do the trick.
If you want the Mod to be "Missile Carrier", extract the rar in the BtS Mods folder but add a folder called Assets to contain the Art & XML folders. Whatever works for you.
EDIT: SORRY! Wrong answer: the structure is valid as described above, the Art folder can be placed as such, but the XML files will have to be merged. For every XML file, you should copy the corresponding one of BtS and then edit them with a text editor (like Notepad). Replace the existing entries and add the new ones.