I just checked the plugin code... It's only available to users who have the "unfiltered_html" capability, but in Multisite only superadmins have that ability. ( http://codex.wordpress.org/Roles_and_Capabilities#unfiltered_html )
I edited the plugin code so that the permission level is "publish_posts", and switched the sanitizing filters to:
/* Remove the filters which disallow HTML in term descriptions */
remove_filter( 'pre_term_description', 'wp_filter_kses' );
add_filter( 'pre_term_description', 'wp_kses_post' );
remove_filter( 'term_description', 'wp_kses_data' );
add_filter( 'term_description', 'wp_kses_post' );
I'm still pretty new at this though, so if anyone notices any mistakes in the above, please let me know!