Just change msg $nick to msg $chan.
As to it not responding, make sure you have only that script there and make sure you're using valid calculation requests. Try:
!calc 1+1
Considering you said the bot was replying with an error message (which wasn't included in what I wrote for you), most likely you have another script loaded and it's giving you the issues.
Also, you can usually use:
msg nick,nick,nick some message to send a message to multiple nicks. There is a limit on the number you can send to at once though and that depends on the server. ** You may need spaces after each comma in that. I can't test right now.
You might also want to change the !calc line to:
elseif ($1 == !calc) { msg $chan $2- = $calc($$2-) }
That will show the requested calculation along with the answer. And, per jaytea, we shouldn't really need to remove anything and I tried a few tests which don't give problems, so that should be fine.