Open your mIRC Scripts Editor (Tools | Scripts Editor, or press Alt+R).
Choose Aliases tab and type in one of the lines illustrated on the following screenshot.
/mp3 /dde mplug announce /F12 /dde mplug announce preset 1 /sF12 /dde mplug announce preset 2
The first one will announce song info using the current AMIP preset when you type /mp3 in mIRC. Second will announce when you press F12 using present #1. The last one announces on Shift+F12 using preset #2.
Please note that you need to have a sane command in the preset which announces to mIRC, like the default one:
//describe $active np: %name [%min:%sec~m/%br~kbps/%sr~kHz]