I am a moderator for a group and the messages posted are not showing up on the group site.?

Are you sure your Group's Archives are turned on? To check, click on Management on the left-hand side of your Group's web page. Then click on Messages under Group Settings. Then click the Edit link next to Posting And Archives. Under Archive Options, either Anyone or Members should be selected; if it isn't, select one of those two, then click Save Changes (towards the lower-right-hand corner). If it is already on Anyone or Members, then fill out the Help Form in crazy_mom's answer.