Raycast help

Hi everyone,

Im in need of some assistance, for a school project im making a travian like rts game. Ive been brushing up on some unity functions but one i cannot use properly is raycast. I wanted to popup a GUI dialog box when i clicked on top of a building, but i cant get the raycast to work. Can anyone explain to me how can i activate a GUI box when i click on top of an object with a interface IBuilding?

Many thanks