Check out the line-in recording tips in MC's on-line help or at this link:
http://www.musicex.com/mediajukebox/how_to.htmlxen-uno is right, you need to play with the two settings to make it work for your particular audio setup. More specifically, you'll probably need to raise the "max gap level" and/or lower the "minimum gap between tracks". If the gaps between tracks on your LP have a fair amount of noise in them, MC will not recognize them as silence so it won't do a split track. Try raising the max gap level one percent at a time until it starts splitting tracks.
Also, use a watch to time how long the gaps between tracks are. If they are about 2 seconds, set the "min gap between tracks" to about 1500 (1.5 seconds). If the gaps are about 3 seconds, set this parameter to about 2000 (2 seconds).
Oh, I forgot to mention the most important thing. The recording volume has to be set right, not only to do auto track splitting correctly, but to get decent quality recordings. Did you use the "auto volume" setting button? Do this BEFORE you adjust the parameters above.