In order to use SketchUp 2014 or newer, yes, as the most recent version supported in the V2 plugin was SketchUp 2013.
It would be completely unsupported, but if I recall correctly, it may be possible to install SketchUp 2013 (you can still get that on the SketchUp downloads
page), install the SketchUp 2013 x86 version of the Maxwell V2 plugin (available through your account on the https://portal.nextlimit.com
download page), and then manually transfer the plugin files over to a SketchUp 2015 x86 installation. But, as I say, this would be unsupported, and even if it works, would be limited to 32-bit, not 64, since there was of course no such thing as a 64-bit SketchUp until SU2015.