Go into you My Account Settings at DW, click on the Other Sites tag, and in the footer section of the Crosspost Settings section insert the following in place or what you have there now*:

This entry was originally posted to Dreamwidth at %%url%%. There are %%comment_image%% comments to this post there.



If you want to create you own text, just make sure to include that weird looking chunk that says comment_image with the percent signs around it. I think that's the part that keeps count of how many comments there are at DW.