sideloading of apps is not enabled on this site : sharepoint online

Are you developing Apps for SharePoint 2013 on a non developer site ? There is a new site template available for developers to develop their apps and test them on a Developer Site Template. You should use Developer site template to develop and test your apps.
If you are trying to deploy your app on a non developer site then you would receive an error "side loading of apps is not enabled on this site SharePoint online".

Side Loading is a hidden feature  which should be enabled on the site to deploy your app. By default this feature is not enabled on any non developer site, so when Visual Studio tries to deploy your app, VS checks if side loading is enabled.
VS calls a method Microsoft.SharePoint.Client.AppCatalog.IsAppSideloadingEnabled and it returns FALLS if it is not a developer site template. However, this hidden feature can be enabled by using Power Shell. Please follow the below MSDN link which explains how to enable them.
http://blogs.msdn.com/b/officeapps/archive/2013/12/10/enable-app-sideloading-in-your-non-developer-site-collection.aspx

Remember side loading of apps is not secure on production sites. It has potential risk of unwanted apps to get added in your site. So never turn on this feature on a production site.

SharePoint Developer Dashboard

SharePoint 2010 has introduced a cool tool for developers to troubleshoot performance issues, the Developer Dashboard. By default the tool is disabled on SharePoint 2010. You can use stsadm or powershell scripts to enable or disable the tool.

Powershell to Enable Developer Tool

Execute the below powershell script to enable Developer Dashboard on your environment.

$service = [Microsoft.SharePoint.Administration.SPWebService]::ContentService
$addsetting =$service.DeveloperDashboardSettings
$addsetting.DisplayLevel = [Microsoft.SharePoint.Administration.SPDeveloperDashboardLevel]::On
$addsetting.Update()

It displays the output window at the bottom after the page loads.

OnDemand Developer Tool

$service = [Microsoft.SharePoint.Administration.SPWebService]::ContentService
$addsetting =$service.DeveloperDashboardSettings
$addsetting.DisplayLevel = [Microsoft.SharePoint.Administration.SPDeveloperDashboardLevel]::OnDemand
$addsetting.Update()

On Demand enables a small button to make it available when needed.

Disable Developer Tool

Execute the below script to disable the tool.

$service = [Microsoft.SharePoint.Administration.SPWebService]::ContentService
$addsetting =$service.DeveloperDashboardSettings
$addsetting.DisplayLevel = [Microsoft.SharePoint.Administration.SPDeveloperDashboardLevel]::Off
$addsetting.Update()