Sometimes when you try to share a placemark with the GE Community forum, by right-clicking on it in My Places and selecting "share/post", you get a message telling you that you have not logged into the GEC forum even though you have already done so. This problem seems to occur mostly if you are using the browser window built into GE itself to view link results, the forum etc.

Try switching to the use of your external browser by going to GE Tools > Options > General Tab and checking "Show web results in external browser".

Also, there is an option to remember your name/password when signing into GEC. If you enable that function while using an external browser, you may be able to subsequently use the GE internal browser for sharing purposes.

After posting, be sure to read When Will My New Placemark Appear in Google Earth?