I believe that the link you gave mislead the other poster. She thought it was to your blog. However, that does not matter. The point is the one katm made.
WordPress.com hosted blogs all run on a multi-user blogging platform wp-MU and it is different software than what wordpress.org runs on. We cannot use plug-ins on wordpress.com hosted blogs.
At the head of this forum is a sticky post titled "Please read me first before posting". That post describes the differences between wordpress.com and wordpress.org.
If you want to be able to use plug-ins and edit your underlying blog template then a wordpress.com hosted blog on this multi-user blogging platform is not for you. And if these are deal breakers for you then you need to hire a web host and download the free standing software from http://wordpress.org