Enter the attic space during the daytime and look for obvious signs of daylight. Anywhere daylight is coming in could be an access point for a mouse.
Inspect points in the roof and side walls for vents, electrical and plumbing for any cracks.
Follow the mouse paths and droppings in the insulation back to outside walls. This may lead to their access point into the attic.
Bait traps with a small amount of peanut butter. Peanut butter stays on the trap better than cheese and reduces instances of traps being set off without actually catching a mouse.
Set traps along the mouse paths in the attic insulation. Set multiple traps, and make note of which ones get the most business. That could help determine the location of the access point.
Check the traps for catches no more than daily. Move empty traps to another location and add bait as needed.