This section illustrates the 2 minutes step by step procedure for adding Magic the Gathering card tooltips to your mybb forum. If the images do not correspond to your setup, you might have a different mybb version installed than mine. Send me an email, and we'll work it out.
First, log in and visit the Templates & Styles tab in your Administration Control Panel. From the left menu, select Templates, and click on the Edit link of the default Header template.
In the textarea that will open add, just like the highlighted bit in the image, the following snippet. Then save the template.
That's it, you're done! All posts containing [mtg] tags around card names and decklists will turn these into nice links that show tooltips when you hover over them!