Here's one way:
Open manage | Database | Relationships
Double click the relationship line that links Contacts to Donations ( You DO have such a link don't you?)
Enable "Allow creation of records via this relationship" for donations.
Create a portal on your Contacts layout that refers to Donations and place the fields you need to log the donations.
WHen you enter data in the bottom blank row of this portal, Filemaker will create a new record and enter the matching Contact ID into this new record for you.