After downloading the manual from the link above, you will need a program to extract rar format archive files. The trial version of WinRar 3 can be download at, as can the freeware command line unrar program (under the menu: RAR | Extras), a 131KB download.

If your current zip / archive manager program can not handle rar files, there are several freeware alternatives, including; 7-Zip (1MB), Filzip (1.2MB), ICEOWS (740KB), IZArc (3MB) and WinImp (314KB).

7-Zip is open source and (unlike a couple of the other programs mentioned) is still being updated and maintained.
ICEOWS is small and has good context menu integration, but rudely takes over file associations without asking when installed; it is easy to change this in the options, but it does not restore the previous associations. Unless your current archive program can re-associate itself from its options (which is not uncommon), you might need to reinstall it if you don't want to switch to ICEOWS.

If you managed to extract the dd_manual.pdf file from the DDmanual.rar archive, but are having a problem opening it, you probably need to install or update Adobe's Acrobat Reader. Version 5.05 is included on DD's install disk 1, and the game install should have offered to install Acrobat Reader. I would expect that version to be able to open the manual, but can not test that (I had to upgrade to v6.0 awhile ago for something else, which BTW will open the manual).

If you are clicking on the link in your browser and it tries to open the rar file directly, try right clicking on the link and selecting 'Save target As' (or something to that effect).