What should I do to atone for the sin of taking money from a wallet I found, knowing that I feared being accused of theft if I returned it, and I have now forgotten the owner's address?

It appears that you knew the owner of the money when you found it. Therefore, this money is not considered a "Luqatah" (lost and found item) Islamically. You must search for him to return his money to him. If you are unable to find him, then donate the money to the poor, or spend it on something that benefits the general Muslim community. If you find him later and he is pleased with your donation, then all is well and good. Otherwise, you must pay him back his money. You should also frequently repent and seek forgiveness for your shortcoming.