Visitor Maps and Who's Online make Feature Image Button disabled

Hi
I just have installed fress copy of wordpress 3.6 for my site with old data base already from 3.3.x version.
The problem is that i found Visitor Maps and Who’s Online version 1.5.8.1 make the Set Feature Image does’nt works.
Any one face that problem please share the solution with me

I use wordpress with arras theme
My site is http://www.vnbuilding.com.vn

Thank in advanced

http://wordpress.org/plugins/visitor-maps/