In order to do so, go to the root folder for Dystopia. This is located at drive:\dir\Steam\steamapps\youraccountname\dystopia (replace drive:, dir and youraccountname with the relevant values for your installation of Steam); It may possibly be in drive:\steam\steamapps\youraccountname\source sdk base 2007\
The .mdmp filename contains the date and time of your crash. In order to determine which .mdmp is relevant to a particular crash, look at the filename. It contains both the date (in international format - YYYY/MM/DD) and the time (hh:mm:ss) of your crash.
The easiest way to get the .mdmp files to the devteam is to attach them to your post. To do so, you have to zip the files up as the forum does not accept the .mdmp extension out-of-the-box.
Once that's done, click the Upload Attachment tab in the post options at the bottom of your the post editbox:
Then click Browse, find your .zip and click Open:
Finally, you'll see the following menu:
Add a file comment - any comment, doesn't matter (though a relevant description works) - and click place inline. You need to do this, or the forum will not upload your attachment properly.
That's it, you're done - just click Submit and your post will go through.