PlayerShowHealth isn't a class.
However, there might be an Armor::OnDamage function. The main way we usually check is by running a console trace. Open a new copy of Blockland, go into singleplayer, do trace(1);, then damage your player and quit. Then read the console log and find the function that was called when you were damaged, remember to keep the same number of arguments that are displayed in the trace.
If Armor::OnDamage really is a function, then here is an example of what you probably would be doing.
function Armor::OnDamage(%this, %player, %that, %andthat, %thatthatthat) //Note the ambiguous variables are because I actually don't know what they are.
{
%player.client.centerprint("\c6Health: \c3" @ %player.getdatablock().maxdamage-%player.getdamagelevel() @ "%", 3); //There is no such 'gethealth' function, the one I know of is getdamagelevel, so we can use that.
parent::OnDamage(%this, %player, %that, %andthat, %thatthatthat);
}
Also remember that you have to package this.