Sorry for my rude answer.
Once downloaded, put the XR-2 zip file (or other files) into your Orbiter folder, rightclick on it, and choose "extract here".
The trick is to let all the addon files go into their dedicated Orbiter subfolder, which will be the case with "good" addons like XR-2, and many others.
Sometimes (rarely) a developer packages his/her addon in the
wrong way, and in that case you should first take a look at its folder structure, just to be on the safe side.
If in doubt, rightclick the addon in your download folder, and tell your unzipper (I use 7-zip) to extract it in "addon_name_folder" (in the XR-2 case, 7-zip will create a "XR2Ravenstar-1.10" folder).
So you will recognize Orbiter folder structure and you'll be able to CTRLX-CTRLV all those files and folders into your Orbiter one.
Can't be THAT tedious!