how to turn on developer dashboard

Using STSADM

Open a command window to the %ProgramFiles%\Common Files\Microsoft Shared Debug\Web Server Extensions\14\BIN directory and enter one of the following commands, depending on the desired display mode.

Mode	Command
On	stsadm -o setproperty -pn developer-dashboard -pv on
OnDemand	stsadm -o setproperty -pn developer-dashboard -pv ondemand

REF:
http://msdn.microsoft.com/en-us/library/ff512745(v=office.14).aspx