Do you have the sprite collection data object in a folder called resources?
Eg. with DemoSpriteCollection that is distributed with tk2d, do this:
demospritecollection/DemoSpriteCollection (tk2dSpriteCollection)
demospritecollection/SpriteCollection_Data/data (tk2dSpriteCollectionData)
needs to be changed like so:
demospritecollection/SpriteCollectino_Data/resources/data (also, rename this to something unique, please).
Now you can load data using the code you're using. If it isn't in resources, you won't be able to load it.