Here is what I am getting if I use a single quote around the server and instance name:
Get-ChildItem : Cannot find path 'SQLSERVER:\SQLRegistration\Central Management Server Group\'Server\InstanceName'\NonProd
\Dev' because it does not exist.
At C:\SQL\CMS_Query.ps1:16 char:24
+ $instanceNameList = dir <<<< $serverGroupPath -recurse | select-object Name -Unique
+ CategoryInfo : ObjectNotFound: (SQLSERVER:\SQLR...com\NonProd\Dev:String) [Get-ChildItem], ItemNotFoundException
+ FullyQualifiedErrorId : PathNotFound,Microsoft.PowerShell.Commands.GetChildItemCommand