The Distinguished Name of your grid certificate is a string identifying you within the CA.
It should be in the format:
/C=UK/O=eScience/OU=[Your institute]/L=[Your department]/CN=[Your name].
If your grid certificate is installed in your browser, the simplest way to find your DN is to access the Imperial DIRAC web portal and copy it from the bottom-right corner of the browser window.
See here for more information about Distinguished Names (DNs).