I am installing a 30 amp single pole breaker into a Square D main lug panel and need to clarify where to attach the ground wire. Do I attach it to one of the neutral bars, or do I need to install a separate ground bus bar to attach it to?

A MLO panel may not have the Main Breaker contained within it, therefore, the neutral bar is for neutral conductors only, and the panel should already have a separate equipment ground bar.

Only in a panel that has the Main Breaker allows the neutral bar to be for neutrals and equipment grounds to connect.