How do I use widgets?

Also how do I get a custom chatbox?

How do I make the custom chatbox display on the bottom of the homepage of my forum?

Since you are using phpbb2, Add the code as mentioned here.
http://www.webartzforum.com/tutorials-f7/how-to-place-ads-affiliates-top-bottom-of-forum-content-t4.htm

I'm using phpbb3.

Then its not possible.

You can add it here
Display>Generalities
Homepage message
Message Content: Add the code here

This will add the chatbox above the forum content on the forum index page.

ok thanks.

do you think I should change to phpbb2?
What are the disadvantages of changing from phpbb3 to phpbb2?
Why can't there be template editing for phpbb3?
What is the link to the green background color for this forum?

Its upto you whether you want to change or not.

There is no such disadvantage as such, only looks change.

Template editing is not allowed in phpbb3, its currently only available in phpbb2 & punbb

Well, you will have to search for the color code
http://www.computerhope.com/htmcolor.htm