being also pretty new to linux mint cinnamon i was playing around with some themes and could not really get everything working as it should.
the big question is what kind of theme are you referring to? themes for cinnamon are different from window themes for example altough they all could be thrown into /usr/share/themes/[themename] or better ~/.themes/[theme_name]
if you check through some of the folders in /usr/share/themes you might notice that there can be several types of subfolders found:
cinnamon, gnome-shell, gtk-2.0, gtk-3.0, metacity-1, xfwm4
i can't tell for sure but it seems obvious that the cinnamon subfolder holds theme-data concerning cinnamon (menu, bars, ...) while the others hold theme-data for the window-decorator (window-borders) and the window composition itself (menus, dropdown-lists, ...).
since there are different themes there are also different ways to select them:
selecting 'menu' -> 'preferences' -> 'cinnamon_settings' -> 'themes' opens a window with 2 tabs, the left one titled 'cinnamon themes' and the right one 'other settings'.
the window-, cursor-, icon- and gtk- themes are selected in 'other settings' so if you dropped a theme containing a metacity-1 subfolder into the theme-folder you should be able to select the 'window theme' in the corresponding drop-down list in 'other settings'.
what i observed is that some themes seem to be broken somehow and are therefore not being listed. it might be worth comparing the file-structure with a selectable theme and see if something is missing. if something is missing it might be a good idea to ask the creator of that theme for further advice.
it would be great if someone could enlighten me with this but afaik someone running linux mint 13 cinnamon could ignore/delete all gtk-2.0 and xfwm4 subfolders as the should be never used.
what i don't really get is the structure needed within the gtk-3.0 and metacity-1 subfolders as this seems to be in charge for a theme being listed or not in the respective selection.