I have been trying to get the Developer Dashboard working with MSS 2010 Beta 1 and having a few problems.  There are a number of blogs out there which claim to make it work but most seem to assume you have MSS 2010 beta 2.  This is the only one I have found so far that actually works with MSS 2010 Beta 1:-

 

Add-PSSnapin Microsoft.SharePoint.Powershell
$farm = Get-SPFarm
$perfmon = $farm.PerformanceMonitor
$level = [Microsoft.SharePoint.Administration.SPPerformanceMonitoringLevel]::On
$perfmon.DeveloperDashboardLevel = $level
$perfmon.Update();