strange error when i import example

and thanks for your help
I download the example of android book android 4 application develop, now I import the project but i see i have in console this error 
 [2012-07-31 12:07:13 - PA4AD_Ch09_MyService] Unable to resolve target 'android-15'

and in the all class i have this: 
 `import android.app.Activity; // for all import pachage`

I have this error:  The import android cannot be resolved
I see this error for all import package
if i create a new project all work well.
when i used android sdk tool 15 all work well, now i don't know to do, can you help me? 
best regads
Antonio

Go to Window menu -> preferance -> select Android --> check SDK location