Should be something like this, although I don't see where $server is ever defined.

Shanee's code (don't forget the = after $null )
 Code:
$null=SetDefaultPrinter("\\" + $server + "\" + $name)


or with SetAsDefaultPrinter(requires UDF)
 Code:
$null=SetAsDefaultPrinter("\\" + $server + "\" + $share)