bdbrown on "How do I resolve this fatal error code"

http://www.marshaottumowen.com

This is my problem:

I was trying to fix my shopping cart so that it would calulate shipping and tax. Somehow, I did something that caused this fatal error and now I can't get into my account.

Fatal error: Call to undefined function is_cart() in /home/exile7/public_html/wp-content/plugins/woocommerce-gateway-paypal-powered-by-braintree/woocommerce-gateway-paypal-powered-by-braintree.php on line 591

This answer:
Use your host file manager or FTP to go to the /wp-content/plugins folder and rename the woocommerce-gateway... plugin folder. That will disable the plugin and should allow access to your admin logon.

was given to anther user who had the same fatal error and it worked for him, however, I do not know how to find the host file manager and I don't know what an FTP is......how do I go to the /wp-content/plug-ins folder to rename the woocommerce-gateway?

I have a wordpress.org account - with one item and a shopping cart so far (that doesn't calculate shipping or tax, btw.) I really need guidance here.