Hm... try removing the = between the variables

Code:

var %user $nick


Also, on the on *:JOIN: add a msg that states you're earning coins and how many you're earning.

Code:
on *:JOIN:#: {
  var %user $nick
  writeini -n rxcoin.ini #michaelukz $nick coins $calc($readini(rxcoin.ini,3michaelukz,%user,coins) + 1)
  var %var $readini(rxcoin.ini,3michaelukz,%user,coins)
  msg # You are now earning coins, you have %var coins
}


Oh, that's your problem, you forgot to include a $ before the calc.