Simply go on the publication page for which you wish to generate a DOI, and press on "More" then on "Generate DOI". That should suffice.
However, some publications cannot receive a DOI code, depending on their type. For example, articles cannot have DOIs generated by ResearchGate as its assumed that they are already published in a journal somewhere so they already have DOIs. To avoid this, simply change the type of your publication, if its too annoying of course. Hope this helps.