Here's a quick step by step to achieve the simple effect I used:
1. Using the rectangle tool

draw your two colored rectangles and align them top to bottom using Object>Align&Distribute or ctrl+shift+A. With both rectangles selected group them (Object>Group or Ctrl+G).
2. Again using the

draw the rectangle with rounded corners. The rounded corners are achieved by dragging the circular "node" when using the

. You should copy the dimensions of your Group from step 1.
3. Duplicate the rectangle from step 2 and select it and the group from step 1. Align the centers of the two objects.
4. Do Object>Clip>Set.
5. Make another duplicate of the rounded corner rectangle and do Path>Object to Path (Ctrl+Shift+C). Duplicate this path. I have changed the color of the duplicate path to make it more obvious what I am doing, but this is not necessary.
6. With both paths selected do Path>Difference (Ctrl+-). This is the basic shape of your highlight and shadow. You can change the fill color, blur, and transparency in the fill and stroke dialogue as suits you to achieve your desired look.
7. For the highlight I have a white fill with blur set to 6 and transparency 85%. I duplicated this object and rotated it and changed the fill to black with the same blur and transparency for the shadow.
8. Here I've just added a darker background so you can see that the blur of the highlight and shadow spill outside the edges of our basic rounded rectangle shape.
9. We correct that by grouping the highlight and shadow with the basic two-tone shape. Then select that group and the rounded rectangle shape, align center and middle.
10. Object>Clip>Set trims the highlight/shadow to the basic shape.
11. You can add your text on top of this, though adding it before the highlight/shadow may give a better result (since the highlight/shadow will appear to affect the text as well as the underlying box. I did it this way for the tutorial to avoid cluttering the basic process.
That's it. There are lots of "shiny button" and "crystal button" tutorials out there if you google them for other ideas on how o make this pop out a little more from the background. Good luck.

For practice you can add a step after 10 that is not illustrated:
10.5. Take a duplicate of your curved rectangle, align it with the finished shape and put it in a layer above the current layer. Change the fill to gradient and use the gradient tool

to make a diagonal gradient with a light colored stop in the top left and a dark stop in the bottom right. Set the layer mode to screen.