These distribution maps are written in the SVG format (Scalable Vector Graphics). You need an SVG viewer to view them.
If your browser is Mozilla Firefox, then you should upgrade to the latest version (free download at Mozilla Firefox), which has an integrated SVG viewer.
If your browser is Internet Explorer version 7, then incompatibility problems with toolbars might be the reason. You should install/update to the latest versions of the toolbars that you have installed.