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.