In the Mortgage interest area of the TurboTax interview, you should make a separate entry for your HELOC (as opposed to your regular loan). Go to Deductions & Credits->Your Home->Mortgage Interest, Refinancing, and Insurance and click on Start or Revisit on the right.
The first thing is that TurboTax will ask you the information that is on the 1098. You must enter the information that is on boxes 1, 2, and 3 on the 1098, because these help TurboTax to determine whether or not the interest on your HELOC is deductible.
Continue, and soon you will come to a screen with the title of "Is this the original loan you took out to purchase this property?" Answer "NO" and you will see some more lines added to the screen, so then answer that it is a HELOC. Then next screen will ask you what you used the HELOC loan proceeds for. Depending on your answers, there may be additional screens.