That's strange that nothing seems to happen when you click the button. Let's make sure that your button is wired up correctly to createExpense in formController.js. Your button should be wired up to the controller using press="{!c.createExpense}". Assuming you have the code from the Quick Start, you should have set these attributes in form.cmp (np refers to your namespace):

<aura:attribute name="expenses" type="np.Expense__c[]"/>
<aura:attribute name="newExpense" type="np.Expense__c"
             default="{ 'sobjectType': 'np__Expense__c',
                         'Name': '',
                         'np__Amount__c': 0,
                         'np__Client__c': '', 
                         'np__Date__c': '',
                         'np__Reimbursed__c': false
                       }"/>

createExpense in formController.js should look like this:

createExpense : function(component, event, helper) {
    // get the value in the amount field
    var amtField = component.find("amount");
    var amt = amtField.get("v.value");
    // display an error if amount is not a number or blank 
    if (isNaN(amt)||amt==''){
        amtField.setValid("v.value", false);
        amtField.addErrors("v.value", [{message:"Enter an expense amount."}]);
    }
    else {
        // clear the error
        amtField.setValid("v.value", true);
        var newExpense = component.get("v.newExpense");
        helper.createExpense(component, newExpense);
        }
	}

 Is your code finding the "amount" field at this point?  You can try to add console.log(amt) after amtField.get("v.value") to see if the value is being captured. This field should correspond to aura:id on the ui:inputNumber component in form.cmp.

<ui:inputNumber aura:id="amount" label="Amount" 
                    class="form-control"
                    value="{!v.newExpense.np__Amount__c}"
                    placeholder="20.80" required="true"/>

If your createExpense code in formController.js is correct, it should call helper.createExpense in formHelper.js.

createExpense: function(component, expense) {
      this.upsertExpense(component, expense, function(a) {
        var expenses = component.get("v.expenses");
        expenses.push(a.getReturnValue());

        // update the list of records in the UI
        component.set("v.expenses", expenses);
        // update the counters
        this.updateTotal(component);
      });
	},

This helper then gets the saveExpense Apex controller, which saves the new record, and updates the app UI via 
component.set("v.expenses", expenses); in helper.createExpense.

@AuraEnabled
    public static Expense__c saveExpense(Expense__c expense) {
        upsert expense;
        return expense;
    }

Hope this helps you find the problem. It would be easier for you to debug the event error if you get this record creation part fixed first. Good luck!