ActiveX/Java Uploader Express does not support multiple license keys. See the Upload Suite Editions topic for details.
To register a license key just specify it as a value of the Uploader.LicenseKey property. Here is how you can do that (note that if you have several keys, they should be separated with semicolons):
<aur:Uploader ID="Uploader1" runat="server"
LicenseKey="XXXXX-XXXXX-XXXXX-XXXXX-XXXXX-XXXXXX;YYYYY-YYYYY-YYYYY-YYYYY-YYYYY-YYYYYY" />
$uploader = new Uploader("Uploader1");
$uploader->setLicenseKey("XXXXX-XXXXX-XXXXX-XXXXX-XXXXX-XXXXXX;YYYYY-YYYYY-YYYYY-YYYYY-YYYYY-YYYYYY");
var u = $au.uploader({
id: 'Uploader1',
licenseKey: 'XXXXX-XXXXX-XXXXX-XXXXX-XXXXX-XXXXXX;YYYYY-YYYYY-YYYYY-YYYYY-YYYYY-YYYYYY'
})