national sewing machine - fabric guide?

hi... I have a National sewing machine from the 70's or 80's. ( i don't see a model number and since it was a garage sale find, there's no manual)

the fabric guide (under pressure foot) keeps popping off when the machine is running. I put it back in... but it pops right back out. I dont see a clasp to attach. Any suggestions?

thanks

Sherry
April 2011