To do this, open the Opus ActiveX plugin configuration, select the "Internet Explorer" line near the bottom, then add .SVG to the list on the right.
(If you're on 64-bit Windows, you have the choice of using 32-bit or 64-bit IE. With current versions of IE, the 32-bit version is the best one to choose.)
Note that this only affects the viewer, not thumbnails. There may be thumbnailers for SVG out there, though.