If you require any plugins you will need to hire a web host http://wordpress.org/hosting/ and set you your own WordPress.org software install to install them.
There is no FTP access and no blogger installed plugin capability on free hosted WordPress.com blogs.
There is no upgrade you can purchase that changes anything at all stated here http://support.wordpress.com/com-vs-org/ FTP access, and plugins are available only for VIP bloggers http://vip.wordpress.com/our-services/ or for those with WordPress.ORG installs.
I am using "Iconic One Pro" and I want to use the font "Helvetica Neue"
Sorry but that's not a free hosted WordPress.com blog. We provide support only for free hosted WordPress.COM blogs being hosted right here on this multiuser blogging platform. You are posting to the wrong support forum.
WordPress.com and WordPress.org have different log-ins and run different versions of themes with same names. If you don't have a username account at WordPress.ORG click http://wordpress.org/support/ and register one on the top right hand corner of the page that opens, so you can post to the support forums there and receive advice from WordPress.ORG bloggers.