I didn't think you could make the switchbar flash, you can change the colours that the channel names/queries/etc. light up when there is activity in mIRC options: ALT+O > Display in the three colour boxes at the bottom. You can also make sure 'Blink icons' is checked.
The /flash command makes mIRC flash when it's minimised to the taskbar. You can't make it flash when mIRC is the active application. A simple example would be:
/timer 1 5 flash example
Use that and then flip over to another window (i.e. this one!) and wait a few seconds, mIRC will flash with the text 'example' in the titlebar.
Regards,
