Custom command to get the MSSQL page life expectancy counter

Iam now using the latest Microsoft Windowszenpack 2.6.1.Under the "SQL instances" component, I created a datasource with the following information but failed to retireive the counter. Anyone could give me some hints on that

Strategy : Custom Command

Parser : Auto

Use Powershell : checked

Script :get-counter -counter "\SQLServer:Buffer Manager\Page life expectancy";

A warning eventcomplains about the access problem :

"Custom Command error: Get-Counter : Unable to access the desired computer or service. Check the permissions and authentication of the log service or the interactive user session against those on the computer or service being monitored."