Clean And Compile/ Build the project again and then try.
If that doesn't work then try reverse engineering to see if the library is really added to your apk file.
- Rename the apk file to have a .zip extension (As it is a normal zip file)
- Extract the files, then try decompiling the classes.dex file and see if your class file is contained within it. I havent tried decompiling the dex file but there are tools outhere for example: http://forum.xda-developers.com/showthread.php?t=2241002