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:
|dnd_dice.mrc||Dungeons & Dragons 3E|
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.