I assume you are in the same directory that you
unbundled the downloaded file? In other words,
you dowloaded some file (probably a "...tar.gz")
file to lets say /usr/local. At this point,
you unbundle by doing somthing like:
tar zxvf NetPBM.tar.gz
...which created some new directory (perhaps
NetPBM). In this new directory, do:
ls -l configure
...what do you get?
Assuming you see a listing for configure, you
should now be able to execute ./configure