You should put the full package name to your activity there. Example:
android:name="com.example.hello.MainActivity"
EDIT
Now that we know you have an empty src folder, you need to add a package that matches your Android package name in your manifest. Right click on your src
folder and New -> Package. Name it "com.example.hello". Then right click on this new package and choose New -> Class.
Name this MainActivity.
You will probably want to read some tutorials on what you need to do to create the code for an activity.