The network chooses you to be an UP if you're not firewalled (ie don't see the brick wall in front of the blue globe next to LW's connection status), have a fast connection & if the network needs UPs. You'll automatically switch to UP mode when the network needs you, providing you have the option enabled - tools > options > advanced > performance > make sure that 'disable ultrapeer capabilities is not ticked.

Do you mean that you don't see any search results? Or do you mean that when you click on a search result to download, nothing happens?