Just poured a pad on concrete it is about a half inch to low how can I add to it I used a broom to finish it out

There are specialty mixes (usually two part compounds) designed for thin overlays. Some of the products are used for bridge repairs and for leveling other highway facilities, and usually expensive. Below is just one of many one types. You can research others depending on the location and function of the slab and whether it will be subject to freeze-thaw cycles. Good luck.
Concrete Repair