Well, lets see ... Im using CS2, so hopefully it wont be too different
Heres my sucky, non-professional method:
First Get the Rounded Rectangle Tool, which is with the vector masks. If you cant find it, press U and the tool will highlight. It probably isnt a rounded rectangle. So you need to right click on it and select the "Rounded Rectangle Tool", which looks like a little rounded rectangle xP
At the top of your screen you should have a few options such as "Radius". This sets the radius of your round corners. Bear in mind that by default it may be set to inches. You can right click on the box to change to any unit you wish. To the left of the radius box is a little down-arrow, which you may also click on to bring up more options for your rectangle like forcing a square shape, or setting a predefined size.
After youve got everything set up, draw your rounded rectangle right on what you want to select. (Hold down SHIFT as you do so to keep it square if you need to) Do not be alarmed when it shows up as a big, rounded rectangle with your foreground color. Thats just fine!
So what we've actually done was create a vector mask. Now of course it doesnt have to be one. Another thing to note is that its already on its separate layer, w00t! So once youve gotten it to the size and shape you want (youll basically have this big, colored, rounded rectangle over your art) Right click on the layer its in, and click "Rasturize Layer". This will just turn it into any old rounded rectangle.
Now get your Magic Wand tool and select the rectangle and delete it! Yup, delete it! But keep the shape that the magic wand selected. And then you should have a perfect rounded square selection.
o.0 Wow that was really long for something fairly simple... well hopefully that helped anyway!
[I took off my usual red font in case staring at the instructions too long would ruin someone's eyes ^_^;]