Without more information, it's difficult to help you...
As the Processing Android wiki says: "If you shoot yourself in the foot, don't complain if it hurts."

You need the libary called android-core.zip, rename it to android-core.jar. It's located in the modes -> android folder.

To point to the android-core.zip that comes with the Processing application,

1. Open the Properties panel for your project (right-click/control-click on the project → Properties)
2. Click on Java Build Path → Libraries
3. Click the Add External JARs... button
4. Locate the android-core.zip file for your Processing installation.
• OS X
1. Enter Command-Shift-G to open the "Go to the folder:" box.
2. Enter "/Applications/Processing.app/Contents/Resources/Java/modes/android" (assuming you installed Processing to Applications and didn't rename the application) and click Go.
3. Select the android-core.zip file.
4. Click Open.
• Windows
1. Browse to the location of your Processing installation.
2. In the Processing application folder, browse to modes\android\.
3. Select the android-core.zip file.
4. Click Open.