You need to set a right padding on the inner panel:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/base"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".MainActivity" >
<HorizontalScrollView
android:id="@+id/horizontalScrollView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="15" >
<LinearLayout
android:id="@+id/panel"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingRight="1000dp"
android:orientation="horizontal" >
<Button
android:id="@+id/button2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Button" />
</LinearLayout>
</HorizontalScrollView>
<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="1"
android:onClick="addClicked"
android:text="Add" />
</LinearLayout>