OK most contractors have started installing a grille with a filter in it rather than inside of the unit, so you may not have one in actual unit.
Can you give me model on the indoor unit so I can tell you where filter would be if you have one. Also pictures of furnace and duct might help e point it out for you.
If you have a paper clip icon on your page you can attach using that link.Or
You can upload the pictures using the tiny pic site .
Here is a link to the site.
Click on Browse , select the file from your folder.then click UPLOAD NOW.
They will give you a quote to enter in the box to verify you are a human.Enter the quote and click UPLOAD NOW
You will see your picture on right side of screen.
Then copy the link URL for email & IM
Paste that link here on the site.