If in the .pg code for the problem you set
$showHint = 3;
the checkbox for the hint will show up after the student has submitted 3 attempts. The default is $showHint = 1;
In general I find a good way to search for specific information on the webwork.maa.org site is to use Google's advanced search and restrict the domain to webwork.maa.org. For example searching for "HINT site:webwork.maa.org" will bring up links to the above info and a lot more.