How can this mean "They find THE money" when there's no definite article in the sentence?
I was wondering that myself. Any Portuguese speakers know if that is correct?
It cannot mean that. It is a mistake. I am reporting it.
Is there a difference between "Eles acham dinheiro" and "Eles encontram dinheiro"?
Nope! They are synonyms. :D
Isn't past tense more appropriate here? As far as I know present tense CAN also have a past or present continuous meaning.
Without context, we do not know. It could be present as in repetition: «Sempre que vão ao banco, eles encontram dinheiro no chão.» = "Whenever they go to the bank, they find money on the floor."