If installing adobe flash player to your standard browser does not work, try the following.
Adobe Flash player install instructions for webkit (mnemosyne uses webkit, an open source browser):
Installing Flash for Other Browsers on WINDOWS
2. Select Windows 7/Vista/XP/2008/2003/2000 from the drop down menu
3. Select the radio button that says Flash Player for Windows - Other Browsers
4. Click on Agree and install now
5. Follow the on-screen instructions until Flash is installed
If you have already installed Flash using the method above and are still unable to view Flash content in Mnemosyne, your Flash installation may be corrupt. Please do the following to perform a clean installation of Flash:
1. Click Start > Control Panel
2. In Windows XP, open Add/Remove Programs. In Windows Vista or 7, open Programs and Features.
3. Locate Adobe Flash Player Plugin then select it and select Uninstall.
4. Allow the uninstallation process to complete, then reinstall Flash Player as per the instructions above.
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Installing Flash for Other Browsers on MAC
1. Visit the Flash for Other Browsers page
2. Select Macintosh OS X 10.4 - 10.6 from the drop down menu
3. Select the radio button that says Flash Player for Mac OS X
4. Click on Agree and install now
5. Follow the on-screen instructions until Flash is installed
6. Start Steam and test the browser
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Installing Flash for Other Browsers on LINUX.
Don't know if it works on Linux, but it probably will. Please let me know.