Downloading apps to ipod touch?

Okay so I've had my ipod for quite a while now but I usually download apps from my ipod with wifi orfrom my fiancees computer. I have downloaded them from my own computer in the past though. Ihave my ipod set to manually manage everything, yet it wont let me put my apps on my ipod? i went to my ipod and the apps tab where you normally select the ones you want to put on your ipod, but it wont let me put them on without completely deleting everything off my ipod? this makes no sense seeing as i have it set to manual and not automatic. Can someone please help me out??