If clicking on "Search..." in the Help menu opened the mIRC website, this means that your copy of mIRC did not come with a help file. That's fine.
Click on "Index" at the top left of the web page.
Then search the page for "Do not disturb".
To search a web page, you would normally press the keys "Control" and "F" together. You can use this key combination to search any web page in the future.