WP Mobile edition, at least version 3.1, is looking for the specific directory "carrington-mobile-1.0.2" that installs with the plugin. The directory never gets copied into /wp-content/themes/.
You can just copy the directory yourself and then activate the plugin directly.
If you want to use a newer version of the Carrington Mobile theme, such as v1.2, then you may want to change the following line in the plugin's wp-mobile.pfp file:
If you remove the -1.0.2 then it will point to the normal theme installation directory for Carrington Mobile.