Where should I enter the pattern when instructed to report a 2-mile left base?
I'm assuming you're more interested in the base-or-downwind question rather than the where-is-the-2-mile-base question.

Theoretically you're supposed to fly the published procedure unless told otherwise. In this case the procedure would probably be a standard traffic pattern.

Being told to 'report' means only that, you should report it when you're there.

This is not to be confused with the (in my experience) more common instruction "Enter and report a 2 mile base for runway 16", in which case you head for the base and not the downwind, and you report it when you get there. To argue ad absurdum, the clearance "Cleared touch and go, after the touch and go, make left traffic, report mid-field downwind" doesn't mean you should head straight for the downwind.

As always, when in doubt, ask.