It was pointed out to me on the Linux Mint IRC help channel that the Search function is not working properly in Software Manager. I, and others from IRC have reproduced this bug.
I have tested a fresh install in Virtualbox. Linux Mint 8 Main Edition, Up-to-date by default mintUpdate levels.
Using the "Search" button in Software Manager, you are presented with a "Find Software" search box. You can select to search Linux Mint, GetDeb.net, or APT.
Under the "Linux Mint" tab neither the "Search for a .mint application" or "Go to the Linux Mint Software Portal" produce any effect when clicked. In the "GetDeb.net" tab, neither the "Search for a .deb package" or "Go to the GetDeb portal" produce any effect. Under the "APT tab, neither "Go to the Linux Mint repository" or "Go to the Ubuntu repository" produce any effect.