First of all thank you for your respond. I guess I found something and is not the fault of the iframe because it responded and show the white page as it should and also the border. I also try changing the url to my website in show nicely.
But I think the main cause is I install the SMF in the wrong folder, because from what I've tested it should be in my main domain and not in the external folder. This happened because I'm using an add on domain, which made my main domain (see Number 1 - below) is already in a folder, and then I installed the SMF in another external folder which is Number 2 below.
I have poor english, so pardon me if my explanation hard to understand, since my situasion also complicated to explain :)
But I think I figure it out and so far I think what cause it:
Number 1: My domain currently running using an add on domain in a hosting that belongs to another domain (publichtml > http://www.anotherdomain.com).
This make my domain located inside a folder that I'm currently running my WordPress that I'm using, which is: http://www.mydomain.com. So it will look like this: (publichhtml > folder titled "mydomain")
When I install the SMF forum, I've installed it outside my http://www.mydomain.com. Because I made a subdomain (see Number 2 - below), which also created another folder titled "forum". So from publichhtml view both folder are separated and position equally or like pair.
Number 2: My SMF forum is in my sub domain: http://forum.mydomain.com/index.php
So finally I erase the SMF forum and create a folder inside "mydomain" folder which is now look like this:
publichtml > folder "mydomain-my main wordpress" > folder "forum-my SMF app"
Guess what? Now it worked but this make my website slower just a bit. So, I'm planning to purchase another host and move "mydomain" and "myforum" to that host.
Please give your views on this and thank you.