Depends. In the Windows client (NOT Semagic), hit ctrl-J or go to LiveJournal->Select Active Journal. Choose the one you want to post to, then post as normal. (Make sure it got set back before you make your next post - if you kept the default settings, it should do this automatically.)
In Semagic, select it in the "journal" drop-down at the bottom. If you can't see that (on the same line as "post entry" in mine), go to "View" and check "Active Journal", then use that drop-down.
If posting from the web site, you need to be logged in to the site itself (not just inputting user name and password onto the form) and you need the full update page, not the quick update: http://www.livejournal.com/update.bml?mode=full - reached via the "..." next to Update Journal link, or at the "For more options, click here" link on the shorter update form.
On the full update form, if you are logged in when you load it, the "journal to post in" will be the first item in optional settings (below the main text box and first update button) and will be a drop-down with your journal and all the communities to which you have posting access.
There are some other considerations in the FAQ (which is a bit general about clients, blast it):
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timeasmymeasure
no subject
In Semagic, select it in the "journal" drop-down at the bottom. If you can't see that (on the same line as "post entry" in mine), go to "View" and check "Active Journal", then use that drop-down.
If posting from the web site, you need to be logged in to the site itself (not just inputting user name and password onto the form) and you need the full update page, not the quick update: http://www.livejournal.com/update.bml?mode=full - reached via the "..." next to Update Journal link, or at the "For more options, click here" link on the shorter update form.
On the full update form, if you are logged in when you load it, the "journal to post in" will be the first item in optional settings (below the main text box and first update button) and will be a drop-down with your journal and all the communities to which you have posting access.
There are some other considerations in the FAQ (which is a bit general about clients, blast it):
http://www.livejournal.com/support/faqbrowse.bml?faqid=76