Didn't receive a form 1099-R. where do I enter the amount i took out?

If you took a distribution from your 401k, then you should have received a Form 1099-R.  Since you did not get one, you need to contact your 401k administrator to request Form 1099-R.  You cannot enter distribution information accurately in your tax return without the details that will be shown on the 1099-R.

If you have online access to your 401k, you may find the Form 1099-R there.

If the distribution was taken as a loan that you are paying back through your paychecks, then you should not have received a Form 1099-R and it is not reported on your tax return at this time.