Here is a simple json parser: https://gist.github.com/octan3/1125017
$code = "static function parseJSON(json) {return eval('(' +json + ')');}"
$JSONUtil = (Add-Type -Language JScript -MemberDefinition $code -Name "JSONUtil" -PassThru)[1]
$obj = $JSONUtil::parseJSON($jsonString)
-PassThru will give you an object (actually two objects; you want the second one) that you can use to call the functions.
You can omit it if you want, and call the function like this:
[Microsoft.PowerShell.Commands.AddType.AutoGeneratedTypes.JSONUtil]::parseJSON($jsonString)
but that's a bit of a pain.