where do l plug into a 1990 honda accord to find out the engine codes

uh no where because its from 1990. usually an engine diagnostics

Try Pulling The Ashtray out and Looking Behind there

you need to pull back the passenger side carpet and the computer is there. Turn the car onto aux and then you will see a red blinking light count the blinks and thats the trouble code. One long code followed by short codes would be 10 plus the short codes. Hope that helps