Most of the collectibles (food pods) in this game are in the wide open and are very easy to get. You can track your progress for the level by going to the pause menu or by going to the level select screen and putting your cursor over the level. Two of the collectibles in the game you can't get until later because you need the 3rd and final upgrade for the Hot-Enougher. Once you have this upgrade you just have to jump at the collectible and hold RT and it should melt the collectible so they unlock. It is important to get all the collectibles because not only does it give you a 100G achievement but it gives you a weapon that kills the final boss in 1 hit.
If you are having any trouble, use this guide:http://www.xbox360achievements.org/forum/showthread.php?t=20...
All credit for the guide above is to x360a and David x360a.
Hope this helps!