Go to windowsphone dot com, to the how-to section, then scroll down until you see icons on the left. 4th one is Sync Your Phone. Click that to go to the page where you can get the app you need.
All the Windows Phone apps shown at the top are for phones with Windows Phone 8. Your Lumia 900 is Windows Phone 7.8, but you can use the Zune software (at the bottom of the page) to connect to your computer.
I apologize for not providing links, but the Ask a Question forum does not allow links due to problems they've had with spammers.