Under Layer Style(Layer > Layer Style) add a Gradient Overlay and Stroke blending options to your icon text layer. Then set the text layer's blending mode to Darken.
We're going to give our transparent plastic buttons some color here. I think it creates a better effect than just using plain white (ala the 'iMac' Web site). At the end of this tutorial though, I'll explain some variations you can get; including fully transparent.
Finally, apply a Bevel and Emboss. Depth should be 200%. Direction should be Up. Size to 5 and Soften to 0. Set the angle to 90 degrees as well. Change the color of the shadow mode to the same color as your stroke. Now add whatever text you want onto the button to finish it. Apply a light drop shadow onto the text and you're done.
A straightforward plastic button with several variances on creating highlights and sheens, it's not difficult to reproduce. However, whenever you're trying to create a glass or plastic fx, think about where the light is coming from, and the shadow dropping to. Anyways, enough from me, let's get on.
The first step is to make a new image, about 200x200 px. depending on how big you want your nav to be. Use the square selection tool to make a rectangular selection like the one below.