Assuming that the LPs have been converted to MP3 (or another format that MusicMatch can understand), open MM, choose File > Add new tracks to Music Library. You will need to point MM to the folder containing your MP3s.
Then drag whichever files you want to the Playlist window (or right-click them and choose Add to Playlist).
Finally, click the Burn icon under the playlist window, or choose File > Create CD. No need for Roxio - MM can do it all.
It's all there in help, and even more info is available at the MM website.