For some reason, you can't set focus on tabs from the "on dialog" init event. To reproduce the bug open this dialog:
Code:
dialog bug {
  size -1 -1 120 65
  tab "one" 1, 5 5 110 60
  tab "two" 2
} 
 
On *:dialog:bug:init:0:{
  did -f bug 2
}

Currently, only a /timer 1 0 did -f ... gets around the problem. Please fix this bug in the next version.