Most blogs will display somewhere a "Trackback URI", which you copy, and then paste into this field when composing a new entry. For example, this poston your blog has at the bottom of the post the following information:
This entry was posted on Tuesday, July 5th, 2005 at 9:16 am and is filed under Commentary. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
I've removed all the links except the trackback, to make it clear which link you would copy-and-paste.