Note: This relates to all NPAPI plugins.
For both Opera and Chrome, type the following into the address bar:
Activate and restart your browser.
Update 18 March 2016:
The option is still there in Opera, but it no longer works. Silverlight will no longer work with Opera web browser or Chrome.
To get around the problem, use IE Tab http://www.ietab.net. Works in both Opera and Chrome.