Here is some information that may help (if you have previously downloaded the app before)
Option A--Purchased from Google Play Store/Android Market
1. On your device, open the Google Play app.
2. Click the menu button on your phone.
3. From the options that appear, select "my apps."
4. At the top of the page, you will have the option of viewing installed or all apps. Click the button that says "all" to view all apps.
5. Scroll to find the app.
6. To the right of the app title, click the icon that says "free" to install it.
Option B--Purchased from Amazon
1. Open the Amazon Appstore App.
2. Click on the menu button on your phone.
3. Form the options that appear, select "my apps."
4. At the top of the page you have the option of viewing new, installed, or all apps. Click to view all apps.
5. Locate the SparkPeople app from that list and click to install.