Are you sure you have the latest reference of nhtmln.dll? Because here it states clearly how to use the back/forward/etc funtions. it can be downloaded
here.
Here's an example:
dialog test {
size -1 -1 400 400
box "test", 1, 5 5 390 390
}
on *:dialog:test:init:0:{
var %hwnd = $dll($nhtmln,find,test)
dll $nhtmln attach %hwnd
dll $nhtmln navigate www.mirc.com
.timer 1 5 dll $nhtmln refresh 0
}
Open the dialog, let lt to load, and after 5 seconds the page will be refreshed.
The 0 parameter after the word 'refresh' is ignored by the dll, so you can put there whatever you want. instead of 'refresh' you can put 'back', 'forward', 'stop', 'search' or 'home'.