You can use this link: http://na.blackberry.com/eng/services/devices/#tab_tab_update to reinstall any missing applications or reload your OS. Make sure you're using IE because other browsers might not work.
Or you can just download the Docs 2 Go app from DataViz directly here.
To mods: The dataviz site is really the only place to redownload Docs2Go that comes preloaded on a blackberry. I understand this is third party in a sense, but VZ doesnt offer a way to redownload this preinstalled software. Please look at it before deleting a link that is helpful and not harmful to an OP.
Thanks,
{link reinstated since this is Blackberry standard software}