Hi eden
If you want to find products by SKU in ATUM's Stock Central, you just have to open the search columns dropdown beside the search box and choose the SKU column. Please note that every inventory has its own SKU field (when using the Multi-Inventory add-on), so if you try to find inventories by SKU, all of them should have that SKU filled or won't show in the results.
Best Regards,
Salva.