Go to the site and examine it's licence. It's easy to get to, there is a link on the page you (tried) to link to:
http://www.jqwidgets.com/license/[
^]
So depending on what your project is, it may be free to use or not - we can;t say, it depends on what you are doing.
If you aren't sure, contact them directly, and they will give you an absolute "yes, you can" or "only if you pay us".