Top-left in the toolbox. In 5.0 it shares that spot with several other selection tools, so you may need to click'n'hold until it appears (or, even better, take jimoblak's advice and just press 'M').
If, by 'rectangle tool', you're referring to a tool that draws a rectangular shape rather than a rectangular selection, it doesn't exist in 5.0, sorry. You'll need to upgrade to at least version 6.0 for that, but there's no reason why you can't just use the marquee tool and use Edit > Fill or Edit > Stoke to color it. Or, if you need to use a vector shape, create a marquee, then use the Path palette to convert it to a path.