in the
res folder - make a copy of the folder first and rename it to "res_sik" as you might have to keep it if you want to revert anything
i.e. you downloaded public-jackard.res.zip (contains Zhivago):
http://www.mediafire.com/?wk2zgl77bbbzurqunpack the zip, you'll find a res folder inside - drop it over the Ender_xx/res folder and copy all files by overwriting, then start Ender as usual - you should see a another char login screen and the "new" avatars now - if you don't want it anymore just rename res folder to "res_jackard" and your "res_sik" back to res again...
that way you can try out new res files very handy...
Take a look into res folder structure that will help you to understand how and where to put in single resmods...

Alternativly you can make a folder in Ender called "custom_res" and copy the contents of the unpacked res folder there...works too.
Be aware that having a custom res file (i.e. for a cupboard) in Ender res folder already and having another custom res file for the same type (i.e. cupboard) in custom_res folder the one in custom_res overrules the one in res folder...