Can King Macar the Gold-Cursed be tapped without attacking?
You can't just tap your creatures whenever you want. You can only tap it at one of the following times:


When the game tells you to tap it (like when you attack).
When a card tells you to tap it (like Twiddle).
When you can tap it to pay a cost (like Endless Obedience).


Cards that would allow you to tap King Macar include spells with conspire and convoke, cards that grant tap abilities (that has a couple of false positives), and cards that tap creatures as an effect.