Add an action buttons to my custom tool window

Use ActionToolBar, see http://www.jetbrains.org/intellij/sdk/docs/basics/action_system.html#building-ui-from-actions. You can use base class com.intellij.openapi.ui.SimpleToolWindowPanel and call setToolbar.

Thanks, it worked.

I have a follow-up question. I'm trying to make the action toolbar to appear horizontally without a success :|

This is what I've tried: 

public class SettingsPanel extends SimpleToolWindowPanel {

    public SettingsPanel(Project project) {
        super(false, true);
        final ActionManager actionManager = ActionManager.getInstance();
        DefaultActionGroup actionGroup = new DefaultActionGroup("ACTION_GROUP", false);
        actionGroup.add(ActionManager.getInstance().getAction("deployAction"));
        ActionToolbar actionToolbar = actionManager.createActionToolbar("ACTION_TOOLBAR", actionGroup, true);
        actionToolbar.setOrientation(SwingConstants.HORIZONTAL);
        this.setToolbar(actionToolbar.getComponent());

It looks like this:

Thanks again :)

I found the issue.

The first parameter of the super class (SimpleToolWindow) is boolean vertical.