Go back to the source ;-). Ask the one who gave you the RAR file to unzip it, restore the BAK and just deliver the (detached) database files (both MDF and LDF).

Btw: if it is a regular SQL backup you can easily install SQL Server Express. Like most Microsoft applications this is a next-next-finish installation to get you up-and-running. With the Express installation you can restore the BAK file yourself.