When you enter your exercise minutes, is that per day or per week?

Per day.

How do I reset the activity times or delete them?

Just tap on the ones you want to remove.

How do I switch between kJ and kcal?

Tap the big number at the very bottom (where it says total).

How do I set my carb limit to 20 grams?

Select the Expert Checkbox on the right and use the Net Carbs field.