The connectors are not interchangeable therefor you wont be able to plug an ATA to a SATA. If I am not mistaken, you need the enclosure's connector to look like this right?
If so, then yes, you will need to replace the HDD enclosure for a IDE/ATAPI connector, similar to this one, as this is for the macbook pro range.
One more thing, the enclosure that you bought, is it a 9.5mm or a 12.7mm one?
Add a hard drive to your laptop by replacing the optical drive with a second hard drive. Compatible with all MacBooks (excluding Early or Mid 2009 MacBooks, MacBook Unibody and MacBook Air) and all 15" MacBook Pros (excluding Unibody). 9.5 mm Internal ATA — 46 available at 39.95 each.