how do i use the cicular message on my alliance page?

Click on the Alliance button located under your Empire Menu on your tool bar. After you are on your alliance page, you should see a button for "Send Circular Message", it should be located just under the info/stats of your alliance.

if it is not there it means whoever your alliance leader is did not give you rights to post yet and need to pm him/her to fix it under the manage alliance section