There are a couple of things at play here.
First, adding a digital signature is always allowed in Acrobat, but only allowed in Reader if the PDF file has been Reader enabled for signing. To use Acrobat XI to Reader Enable a PDF file for signing you would:
- Select the File > Save As Other > Reader Extended PDF > Enable More Tools menu item
- Click the Save Now button on the Enable Usage Rights in Adobe Reader dialog
- Select a file name and location, and then click the Save button on the Save As dialog
That would allow the file to be signed when opened in Reader.
However, I did mention that there are two items of consideration. If you are using a Pre-release (beta) version of Acrobat then signing will not work in a release version of Acrobat or Reader. When you said you are using a "trial" version I just want to make sure you mean the released version of Acrobat XI being run under the trial license where you don't have a serial number as opposed to a beta version that made available prior to the release date.
I am using Acrobat XI Pro Trial not Beta and I found that the option "Enable More Tools" is greyed out (disabled) if Editing the PDF in "Edit Form Fields" mode. It is available if you are Content Editing.
Thanks for you help,