Hi. Can you tell us which version are you using? Also is your rtw path recognised at least or is it blank?
Earlier builds of xc5 have a slightly broken installer and to be honest it's not yet fully playable.
If you want to try it online with a friend, I'd recommend the final version of xc4 as it's the most stable and fully fledged version.
http://www.wmwiki.com/hosted/cv/xcfiles/XC_4%20v1.12.exe
If the problem persists with this version, you may have a custom steam library location, which unfortunately the installer does not support (yet).
Finnaly, there is always a possibility that your RTW installation is bugged and needs to be redownloaded (not just verified, downloaded from scratch).
P.S: A manual installation is possible, but unless you are at least somewhat experienced with tagged files, I'd recommend against it.