Thank you very much for your quick reply. I am using the lastest sencha command I believe (downloaded yesterday). And i have tried both commands 'sencha app build native' and 'sencha package build packager.json', both no success and no error.

Here is the my packager.json file
{
"applicationName":"testapp",
"applicationId":"testapp",
"versionString":"1.0",
"iconName":"resources/icons/Icon~ipad.png",
"inputPath":"D:/Ming/websites/widgets/sencha-touch/testapp",
"outputPath":"C:/Users/Ming/build",
"configuration":"Debug",
"platform":"AndroidEmulator",
"deviceType":"Universal",
"certificatePath":"debug.keystore",
"certificateAlias":"androiddebugkey",
"certificatePassword":"testapppass",
"sdkPath":"C:/Users/Ming/android-sdks",
"androidAPILevel":"15",
"orientations": [
"portrait",
"landscapeLeft",
"landscapeRight",
"portraitUpsideDown"
]
}

this file is inside the application folder, the debug.keystore file is in the same folder too.
Thanks again for your help.