On the class design screen there's an ordnance page, you specify what the default load out is. When you give the load ordnance command it will attempt to fill the ships magazines to that default.
You do need to have the missiles built and at that colony of course.
Another option is to load missiles manually from the individual ship window.
There's a few options there for selecting which ship to transfer from, then clicking on the missiles to transfer.
Criticism of the design?
It's pretty slow for magneto plasma, quite heavily armoured, weapons load out seems anaemic. What percentage of the design is box launchers?
Your engines are very efficient though which explains the speed.
What is the design intended to be used for ?
That speed and range might make it a good escort for freighters, though I would minimise the armour, add more box launchers for a good alpha strike, and find some way of increasing fire control range, maybe remove the active and then accompany the task group with a dedicated sensor variant.
But that's just my personal view, this ship might meet your goals as is.
Edit: Ostia mentions magazines, not particularly useful for box launchers though :p