you should find your own key from android source tree.
dir like:

android/build/target/product/security$ ls
Android.mk  media.pk8  media.x509.pem  platform.pk8  platform.x509.pem  README  shared.pk8  shared.x509.pem  testkey.pk8  testkey.x509.pem  verity_key  verity.pk8  verity.x509.pe


move your apk file into src 

1  mv  xxx.apk src/

2  ./run.sh src/xxx.apk 

signapk.jar , is build by ourselves
1  go to android src dir 
	android/build/tools/signapk$ ls
	Android.mk  SignApk.java  SignApk.mf  test

2  mm build signapk.jar