By looking at your screenshot, it looks like you are missing a required prerequisite. You could try to install that prerequisite and then retry installing RadControls.
You can find out the exact prerequisite by using your mouse to hover the over the "This product cannot be installed" label. We are working on making that information more visible in one of our next web installer versions.
Please do not hesitate to contact us if you experience any further issues.
the Telerik team