[Brad]

In your ftp client use the following details (how you set them up depends on your client).

Server: ftp.yoururl.com (this is correct most of the time, check with your host if you're unsure)
Username: Your username (again check with the host if unsure)
Password: Your password (again...you get the idea. )
Port: 21 (most of the time it's 21)

Most of the time in ftp applications the local file system is displayed on the left of the screen and the remote file system (your server) is on the right. In the remote file system browse to this folder: /public_html/forum/

Most of the time /public_html/ is the webroot, and /forum/ is where the vBulletin files reside. On your server it may be located elsewhere...but you should be able to find it with little effort.

Once you locate the correct folder on the server do the same for the local file system (find the file you wish to upload in other words). Once you locate the file you wish to upload left click on it and click "upload" (most ftp clients for windows (I'm assuming that's what you use) work like this...at least the ones I've seen).