Make sure you have the latest version of mIRC (6.15 at the time of this post) and then press ALT+O (Or go to Tools > Options in the menu in mIRC). Then, on the navigation bar on the right click on the cross next to 'IRC' and under that appears the word 'Options'. Click on that. On that screen there are several options, but under 'Flash on:' check the box saying 'Channel message'.
If you only want the flash to activate on one channel, right click that channel name in the switchbar (the bar where the channel names appear0 and click on the word 'Flashing' in the menu.
Regards,
