There are two ways to use the bot: default or standalone mode.
Type /loadbot to run the bot. This will launch a second copy of mIRC with the dice rolling events enabled. You do your chatting from the original copy of mIRC, and the second copy (the bot) handles the dice rolls. This arrangement is necessary for technical and practical reasons.
The dice rolling events are enabled in the currently loaded copy of mIRC. You should only use this mode if you plan to use a different IRC client (e.g. Trillian) to do your chatting from, and use mIRC only as a bot.
Follow the link for the dice roller you're using:
Some IRC servers dislike bots and/or multiple connections. This information should be readily displayed in their MOTD. Usually it's best to connect to a different server with your bot than the one you're connecting to with your main mIRC.