Plugins installation;

Open The folder you used for your bukkit server.now, you should see a plugin there called "plugins" open that.(IF YOU DONT HAVE PLUGINS FOLDER, MAKE ONE!!!!)drag the plugin you downloaded in there and i mean the WHOLE file you downloaded. done!!!! run your server and ENJOY!!!!!!

mods installation;

there are many ways. im gonna tell you guys how to install MY mods.

for pc idk how to for mac :P;go to your start and type in run on the search bar. the type in %appdata%. now go to your roaming file,then your .minecraft. now go to your bin,then minecraft.jar. now delete Meta.INF,and drag the conteents of the mod in!!!! enjoy your mod.