There are several ways to enable the developer dashboard in SharePoint 2010 and 2013. I always prefer the PowerShell method, but for the sake of completeness I will show several options here.
Notice that there are the following options for the Developer Dashboard:
SharePoint 2010: The Developer Dashboard will always be rendered at the bottom of each page
SharePoint 2013: The Developer Dashboard icon will always be displayed at the top right corner. It will not be appended to each page.
SharePoint 2010 & 2013: The Developer Dashboard will not be available
SharePoint 2010: The Developer Dashboard will only be appended to a page after clicking on the icon in the ribbon
SharePoint 2013: This mode is not available in 2013 since it reflects the behavior of On now
Enabling Developer Dashboard:
$devdashboard.DisplayLevel = 'OnDemand';
Write-Host ("Developer Dashboard Level: " + $contentService.DeveloperDashboardSettings.DisplayLevel)
Enabling Developer Dashboard using Object Model
SPWebService service = SPWebService.ContentService;
STSADM.exe -o setproperty -pn
developer-dashboard -pv OnDemand
The Developer Dashboard can be disabled by setting the display level value to "Off". As needed the display level can be set to "On", "Off" or "OnDemand" in the above code snippets. This operation requires a farm administrator permission. Make sure you have farm admin access before enabling/disabling developer dashboard.