My anti-virus says the Add-In is dangerous
Norton (and other anti-virus softwares) is notorious for causing issues with Word Add-Ins. Norton relies on a set of heuristics to determine whether something is dangerous. Sadly, one of these is how many times it has seen the same add-in. This is obviously a major issue for smaller software vendors such as ourselves. Norton installs its own plugin to Word that you can't see that in the words of Microsoft "may cause performance issues, the inability to open files within Office, or the inability to install some Office programs."
This Microsoft knowledge base article may help: http://support.microsoft.com/kb/329820
We would recommend that you disable the Norton Office add-in and then try to install the software.