There have been some issues getting the vSphere Web Client and vSphere Client Integration Plugin to work, can not launch the vCSA 6.0 / 6.0 Update installer, or use the "Windows session authentication" check box, mount local ISO file, import OVA/OVF, and more. VMware has an KB on this: KB2114800
The solution did not work.
I had this problem today, with I found annoying, so i took a look at what i had installed on my machine, and found that I had version 5.1, 5.5 and 6.0 installed, They had also been inplace upgraded thru time.
I uninstall them all, and afterward installed det newest, 6.0 Update 1, and using the newest build of Google Chrome (46.0.2390.80 m).
This fixed the problem.
I'm not sure why this works, hope this i usefull.
There is a note i the vCenter 5.5 Update 3a release notes, that coexistence is not allowed.