HOW TO INSTALL?
You have to click on the "Completed order" receipt link for download. This receipt you would have received via e-mail. Then once clicked, a download will begin so just and save the file and remember where you saved it.
You should buy the plugin yourself and just buy the PayPal funds (unless you have no Paypal or are blocked/frozed/banned). You want to not only have a valid license for the software in your name, but also would want to be eligible for support and continuing upgrades. You are likely to get some scammer who will send you a pirated copy otherwise.
This is an opportunity to contact the developer and tell them you would pay them in Bitcoins for their plugin if they would only take them. Show that many already use woocommerce for Bitcoin:
http://wordpress.org/plugins/bitcoin-payments-for-woocommerce/