To install from a minimal tarball, and build up cinnamon the merge commands are:
root # ego profile flavor desktop root # ego profile mix-in cinnamon root # emerge alsa-utils cinnamon xorg-x11 networkmanager lightdm
From a gnome tarball:
root # ego profile mix-in cinnamon root # emerge cinnamon lightdm
Loading through Package:Lightdm is the easiest route to booting cinnamon. You must select cinnamon session in the top right corner of lightdm the first time it is loaded.
If your screen lock is not accepting your password try rebuilding pam:
root # emerge -av1 pam