a quick look in the logfiles:
[27-09-2006][01:41] >> uslacker (email@example.com) has joined ##slackware
[27-09-2006][01:41] << Kick for uslacker by slackboy from ##slackware [banned:It is very dangerous to IRC as root. Please return under a user account. You can create a user account with the adduser command. Once created, logout of root, login to your user account and return to the channel if you wish.]
1) so follow the instructions ..
2) register your nickname as outlined (see freenode website)
and you shouldn't have any problems joining ##slackware