Hi kutu,
If you download many modules, plugins from server; all of these modules, plugins shall be compressed into one file. You must decompress that file and install each module and plugin.
JA Job Board Plugin Payment (Offline Payment) similarly to other addons (http://www.joomlart.com/forums/downloads.php?do=cat&id=138) which is one addon of JAJobBoard, with different installation way for each type.
Ex: In order to install addon JA Job Board Plugin Payment (Offline Payment), you must decompress the file after downloading and copying wirebank.php file into the folder: /components/com_jajobboard/asset/payments/