The SIM is just slid into the SIM card slot.
Here’s the section of the user manual that describes how to insert the SIM card. Take note of where the beveled edge is on the SIM card so that you know it is in the correct way.
It looks like the phone has a SIM card tray that just pops out of the side and you lay the card in that. The tray takes two SIM cards (I think)
Here’s and image taken from the ifixit BLU Tank II Battery Replacement guide, to show where it is.
Looks like there is a small pin hole that is like with most SIM card trays in that you insert a tray ejector tool to pop out the tray.
There should be an image or an outline of the card on the tray which shows the beveled edge and that it has to go face down to connect with the phone
(click on image to enlarge for better viewing)