On a UK keyboard it should be between the semicolon and the return keys. Shifted it's a quote mark (or inverted commas).
Still lost? Go to Keyboard in System Preferences; Under First tab 'keyboard' select the 'Show Keyboard & Character Viewers in menu bar'.
Once you've done this a new icon shows up somewhere on the right hand side of menu bar. Click it and select Keyboard viewer to see what key combinations give you which keys. (i.e try Shift; fn: control: option: and command to see what gives you what)