dimilinowski,
That’s a good question. When you ftp, you FTP to a host (this will generally be determined by the server’s FTP software… well really by the hosting company’s staff) which is generally a domain… mysite.com or https://ftp.mysite.com, etc. In some cases it doesn’t matter what domain you use, as long as it resolves to the appropriate IP. But again, that all depends on the FTP server’s setup.
So enough with the technical stuff… Really all you need to know is that you FTP to mysite.com or https://ftp.mysite.com or whatever (your hosting company can provide this information if neither of those work). From there, you’ll see a directory listing. Click on the directory you want to upload the files to/download the files from (ie /blog). It may take a few clicks to get there, you may have to click on directories such as public, public_html, www, etc. Again, your hosting company can provide this information. Once you’ve gotten to the root of your WordPress installation (you’ll know it’s the root of your WordPress installation because you’ll see wp-config.php, wp-content, wp-includes, wp-admin, etc) then click on wp-content, then on plugins. At this point you can upload the unzipped plugin folder into that directory.
Now all you have to do is go into your admin interface, click plugins, and click activate for that plugin. Make sure you read any documentation for the plugin in case there are configuration options or other setup instructions specific to that plugin.
Hope this helps!