When you install Acrobat XI you are also installing Reader for use in the browser. Reader is "sandboxed", or as it says in the Preferences, it is in "Protected Mode". Acrobat does not understand the concept of running in sandboxed memory (sandbox is a security feature to prevent the bad guy from exploiting a memory error and getting into a memory address where they can do damage to your computer) so to help protect Acrobat users the same way Reader users are protected we just install Reader along with Acrobat, but it is only used in the browser (part of this is because the bad guys use web sites as their primary method of distributing corrupted PDF files that can cause problems).
That said, because you are in Reader the file needs to be Reader Enabled prior to being posted to the web site (or opened in the browser). Once the file is Reader Enabled you should be able to add the text annotations.