If you are using Windows 8 and Internet Explorer 10, you must run Internet Explorer in desktop mode in order to use the Adobe Flash Player (which is required to run the current version of SIMnet).
The SIMnet home page will display the following message at the bottom of the window if you are not in desktop mode:
Click the Open button to switch to Internet Explorer in desktop mode where you can log in to SIMnet as usual.
For more information about running add-ons such as the Adobe Flash Player in Internet Explorer 10, see this page from Microsoft, Manage add-ons for Internet Explorer 10:
Internet Explorer 10 is designed to provide an add-on free experience, and will play HTML5 and many Adobe Flash Player videos without needing to install a separate add-on. Add-ons and toolbars will only work in Internet Explorer for the desktop. To view a page that requires add-ons in Internet Explorer, swipe down or right-click to bring up the Address bar, tap or click the Page tools button, and then tap or click View on the desktop.