Tried a lot but I just can't quite get it, how do I achieve a translucent tab bar like this:
The bottom part is gaussian blurred and has a semitransparent white overlay.
Step1: Select the overlay area on the background (the original image) and apply a gaussian blur of radius 12px.
Step2: Create a new layer, select the same part for the overlay, fill it with white, and give this layer an opacity of 66%
Reproduced with the upper part of your reference image:
You did not define what program or technology you are using. So, while Peter described a nice way to do it with Photoshop, I would like to add that this effect can also be created using CSS3.
Here's a peak into the CSS needed:
Not saying that this is how you should do it… in fact, I personally would use Peter's procedure.