Why i cannot comment with my Acrobat reader 9.5.1?

I downloaded Adobe acrobat 9.5.1 reader software package from Adobe's site. The .deb extension. But i cannot see the comment menu under the tools, in any pdf file that i am trying to comment

That is because Adobe Reader 9 does not have commenting features :)

Adobe Reader XI has commenting and highlighting:

Can I use Reader XI to add comments and highlights to a PDF file?

Reader XI is the first version of Reader that enables users to annotate PDF documents using a full set of commenting and markup tools, including sticky notes, a freehand drawing tool, and typewriter, highlight, underline, and strikethrough tools. You can also use Reader XI to participate in shared document reviews, with team members seeing and building on each other’s comments to speed up review cycles.

Source: http://www.adobe.com/products/reader/faq.html

Check this post for pdf commenting.

If all else fails, try opening your PDF in another program! I'm using Mendeley because it allows highlighting and notes. Seems to work with any PDF, no matter what the properties of the PDF are or what operating system(s) you're using.