I can screen share my wife's Lion iMac using Mountain Lion and the reverse is true. If the computer is not showing up to be selected under shard on the left column of the finder it is probable a connection issue. I would check my home network, restart the the macs and toggle the sharded settings and energy saving settings like wake for network activity. Be sure and set up iCloud on both your macs and turn on UPnP if you have a router for your home network serving up NAT IP Numbers. UPnG will make the port forwarding enteries in you router to allow you macs to connect behind the firewall created by nat addressing.
I am currently having a problem with back to my mac. S
creen shares great the the Lion iMac but Mountain Lion to Mountain on one computer will not connect if the computer is sleeping. It lights the screen on the sleeping computer but never shows on the computer requesting to share. File share always workd. If I wake the computere before I attemp the connection, screen share work as it should. If I try to screen share while the second computer is sleeping, It will never work again until I restart the iMac.