Go to http://www.dreamwidth.org/manage/I stress, there are very good reasons for not crossposting (not to be on LJ at all, not trusting DW with their LJ password, not wanting to need to lock two sites, not trying to keep icons etc in sync, etc) and many people actively don't want to. But if you want to crosspost, I sometimes find it helpful to be able to see comments on both sites, or see that someone has an account on the other site at all.
In the list of foreign accounts, click "details". You should see "edit external account". Make sure the "Display Cross Post Links" checkbox is checked and click the update button. This shows a link from DW to LJ.
Back in account settings, other sites. Select "Always" in "Add the footer to the post on the other site".
Add some text in the first footer text box. I used: "<span style="font-size: smaller;">You can also comment at %%url%% using OpenID. %%comment_image%% comments so far.</span>".
(If you want to, you can set up crossposting from that page, but I can't remember how because I did it once ages ago.)