You dont have a ; at the end of your datablock, and there is no such mountpoint as $1.
so should it look like this
function serverCmdArrow(%client)
{
%Client.player.mountImage(arrow,1);
}
datablock ShapeBaseImageData(arrow);
{
shapeFile = "Add-ons/shapes/arrow.dts";
emap = false;
mountPoint = 1;
offset = "0.1 0.2 -0.55";
eyeOffset = 0.1 0.2 -0.55";
rotation = "0 0 1 180";
className = "itemimage";
}