Install Modern appx applications without Windows Store

So, if you are going to install Modern POS on your machine you will need to do an extra step before actually getting to the install part.
That is about sideloading and how to enable it for lob apps: you have a very detailed explanation of what is it here https://technet.microsoft.com/en-us/library/hh852635.aspx?f=255&MSPPError=-2147217396.

To summarize it:

1. Open the Local Group Policy Editor using gpedit.msc.
2. Expand Local Computer Policy, Computer Configuration, Administrative Templates, Windows Components, and then App Package Deployment.
3. Double-click the Allow all trusted apps to install setting.
4. In the Allow all trusted apps to install window, click Enabled and then click OK.

After you've done this, you can install the MPOS .appx file that is copied from the AX dvd media using the add-appxpackage powershell cmdlet like:

PS> Add-Appxpackage "C:\Program Files (x86)\Microsoft Dynamics AX\60\Retail Modern POS\RetailModernPOS_x86.appx"

Or, you can actually use the powershell script files that come with the AX dvd when installing the MPOS from it. You are getting in the (default) path C:\Program Files (x86)\Microsoft Dynamics AX\60\Retail Modern POS\Tools\ several nice little util files that are doing all the installation for you, including registering the apps for all the users and loading the dependency files.

You will do that in case you either did not configured the MPOS component during AX role installation, or if you are developing on that machine, so the app was uninstalled (in order to run your dev app) and now you want to install that back.