Well, the args are definitely correct.
I just tried this on my server and it actually doesn't work at all, so he will have to go with the way you posted originally. Woops!
package BrickControl
{
function FxDtsBrick::onActivate(%this, %player, %client, %pos, %vec)
{
if(%this.getName() $= "_buyItem")
messageClient(%client, '', "\c6This is a test.");
parent::onActivate(%this, %player, %client, %pos, %vec);
}
};
activatePackage(BrickControl);