The Map Tools need to be loaded in order to view your markings. After you click to load a saved map, look on the right side and see if the map tools are loaded. If not, you may need to click on 'Load Map Tools', wait for the map image to come back up, and then click Open Map in the top right corner to open it manually. You can correct this by allowing popups from MyTopo.com.
Also, ensure you have the latest version of Internet Explorer (9 if you are on Windows Vista or Windows 7). Also, go to www.java.com and click on 'Do I have Java?' to make sure you have the latest Java version. Restart and see if it works.