I have found where the problem was.
When I have transfered the folders and files 'webassist' folder was in the 'install_pm' folder.
but Browse Server button link takes me to this linK
I have uploaded the 'webassist' folder in PowerMessanger folder and change the permissions to 755 and file browser worked fine.
I am surprised that this is not mentioned in 'Getting Started with PowerMessenger' document. Would have saved me a lot of time.