Which OEM HU do you have (what is the model number printed on the front, near the display)?
If your HU has a button labeled "AUX" then the aux kit will probably be plug-and-play. Installation is not too difficult. You can download the instructions from Subaru's site here:
Subaru Technical Information System - techinfo.subaru.com
(select Publication Type "Accessory Installation Guide," optionally filter by your model/year, and use keyword "aux". Or use P/N H6210FG000).