The plugin supports various different PlaceholderAPIs built in to the plugin! You do not need to install anything else to use these, just the plugin.
Each still has the following placeholders. Replace the placeholder <skillname> with the name of the skill, e.g. "mining"
%mcmmo_level_<skillname>%%mcmmo_xp_needed_<skillname>%%mcmmo_xp_<skillname>%%mcmmo_xp_remaining_<skillname>%%mcmmo_rank_<skillname>%%mcmmo_xprate_<skillname>%As for everything else:
%mcmmo_power_level%%mcmmo_power_level%%mcmmo_power_level_cap%%mcmmo_in_party%%mcmmo_party_name%%mcmmo_is_party_leader%%mcmmo_party_leader%%mcmmo_party_size%%mcmmo_is_xp_event_active%%mcmmo_xprate%