Unzip the file you downloaded inside your main mIRC directory. Load mIRC and at the command prompt type:

/load -rs filename

where filename is one of the following depending on the roller you selected:

Filename Roller
diceconf.mrc Standard
dnd_dice.mrc Dungeons & Dragons 3E
altdice.mrc Alternity

A confirmation dialog will now appear:

Click Yes to complete the dice roller installation. Additional dialogs may now appear to allow you to set some options, see the configuration section for details.