Installation: Once a pack has been downloaded, extract the .rar into your osu! Songs directory. All songs are still .zip'd and/or .osz'd inside the pack, so osu! will need to extract the beatmaps itself the next time you go into Play mode.
Do NOT extract the zip's/osz's yourself, or the beatmaps will display incorrectly in osu! and will not function properly.
It should work fine doing it this way.
Source:
Beatmap Packs.