Fixing a transformer inside a metal case
The transformer doesn't have mounting tabs because it's for PCB mounting. Solder it on a piece of breadboard ("Veroboard") with some space around it, and drill 4 holes on the corners for bolts.
Since you're using a breadboard anyway, use it to mount connectors for connecting both the primary and secondary. Don't solder your power supply cable directly to the transformer pins, and especially never solder the power cord!    

Make sure you mount the board high enough above the case so that the transformer pins don't come closer than 6mm from the case.  

edit (re Federico's comment)
Doesn't apply here, but I've seen molded PCB mount transformers with a couple of mounting holes in them to screw them to the PCB. You won't see them on small transformers, like 1VA, but 10VA to 15VA transformers may have them. If the holes are there, use them! The screws will take the strain from the solder joints, which aren't meant for mechanical fixing anyway.