alivia,
Sorry, I'm not clear on your situation. If you invested in treasury securities via your Merrill Lynch account, the interest you received already appears on your Merrill Lynch 1099 combined tax document in Box 3 of the 1099-INT subsection. (They will almost always not issue separate 1099-INT, 1099-DIV and 1099-B forms.) If you have directly imported the tax document, it should already have been entered. If not, or you are entering stuff manually (like I do), you go to the Interest section and enter it there box by box from the form.
If, on the other hand, you purchased your US securities via the government's TreasuryDirect site, you need to log into that site and download the 1099 tax form from there. Then just manually enter the 1099-INT into TurboTax as above.