The best way to do this is to go here, and place something like the following into the "Footer" box:
<A href="%%url%%">Original post on Dreamwidth</A> - there are %%comment_image%% comments there.
This will provide a footer like the one at the bottom of this post (if you're reading it on LJ), and means that when someone comments on DW the LJ readers can come over to see what it is.
You can leave the second box blank, unless you're disabling the comments on LJ.
Original post on Dreamwidth - there are comments there.