Hi Lior,
I'm sorry we didn't see your post.  I hope you figured it out, but for anyone else who may have the same question:
The .lbl file that comes from ZebraDesigner or NiceLabel's label design software does not get loaded onto the printer directly.  You have to export it to the printer using the design software's export functionality.  If you want to push the label to printers using your C# app, I recommend setting up a ZebraDesigner driver as a "Print to File" driver.  that way you can use the design tool to export the printer specific format (in ZPL or CPCL) to a file that you can easily read from C# or copy it directly into your code as a string.
Hope this helps