For users with Adobe Reader X:

If an *unsecured* PDF file is opened in the standalone version of Adobe Reader X, then you have access to the Highlight and Sticky Note tools only.

If the PDF is being views within a web browser using the Adobe Reader X plugin, commenting is not available unless the PDF is part of a browser-based review workflow. To apply comments to any other PDF file being displayed within a web browser, you must save a copy to your computer and open it with the standalone (desktop) version of Reader X.

If the PDF file is secured to prevent changes and commenting, then no commenting tools in Reader X will work, and the two default tools will be disabled on the Comments Pane.

If the PDF file has been granted extended permissions via Adobe Acrobat, then not only will the two default commenting tools be available, but the complete set of markup and measurement tools will also appear, via the Comment and Tools panes in Reader X.

,b>For users with Adobe Reader 9 or earlier:

Unless the PDF file has been granted extended permissions via Adobe Acrobat, NO commenting tools will be available.

When you created a dynamic form with LiveCycle / ADEP Designer, the commenting won't be possible even you added the additional rights to the PDF.
That's because this PDF isn't a PDF in the usual sense.
Its layout is dynamic and can change everytime (showing/hiding fields and subforms, page breaks etc.).
As comments need a fixed location, they cannot be applied at all.