You have 3 options for thanking donors.
1) Automated Reply to Donors - In your account go to “My Menu Options “ and click “Create thank You Message to donors “ . This is automatically sent to donors following each donation.
2) Send message to all past donors - You can also send an email to all donors to date. In your dashboard, click Edit Existing Pages. Under your your fundraising page , one of the buttons (shown below) is Thank Donors . Following instructions on this page.
3) Thank each donor Individually . From same options as number 2 above, click My Donors and you see a list of all your donors. With each donation detail, theres an option to send a Reply to donor. Note this message also shows on your public fundraising page.