Just a handy tip - you don't have to cast enum types in powershell usually. It will do that for you if that's the only constructor (or method) overload that makes sense:

$accessrule = New-Object system.security.AccessControl.FileSystemAccessRule $userName, `
     "Modify",  "ContainerInherit,ObjectInherit", "None", "Allow"

So when I say "only overload that makes sense," what do I mean by that? Take this method for example which has two overloads (meaning it has two different sets of parameters you could use):

void MyMethod(string a, string b, string c)

void MyMethod(string a, enum b, string c)

If you try to call that method with $o.MyMethod("foo", "foo", "foo"), it will pick the first version that takes three strings. In that case, you would have to cast/convert the enum to its native type so that powershell will pick the right method.