In earlier releases I could do that by right-clicking on a document file, going to Properties -> Open With and selecting Abiword, but in 11.10 I can't see Abiword as an option. How can I now make it the default application for opening my doc/docx files?
you can try installing ubuntu-tweak http://ubuntu-tweak.com/ In Admins->File Type Manager in Text and Program Section select the filetypes that you want to change to handle with Abiword
Another way might be to click the Add button and add the executable's path.
See,
xxx@linux-dopx:~/> which oocalc
gives me/usr/bin/oocalc
This path if added by the Add button will appear in that list, and then you can select it.