First of all, the plugin should support translation.
If it is so, the directory where to put po/mo file is defined by plugin, usually they are in a directory /langs or /languages in plugin directory.
That directory is (should be) defined by plugin using load_plugin_textdomain
The plugin you are using has a subfolder cimy-user-extra-fields/langs where you can find the existing localization files.
Your files (cimy_uef-fa_IR.po / cimy_uef-fa_IR.mo), should be putted in the same folder.
To create these files you can use the software PO Edit a powerful software, maybe not very easy to use.
If you already don’t know it I suggest you the plugin Codestyling Localization a powerfull well written WP plugin that will help you trought the entire process of transalitng plugins, themes and wordpress itself.