find out where it installed the "port" command (/sw?)
sudo find / -name "port*"
(look for something in a /bin/ directory or /sw or something.
Add that directory to the PATH environment variable in /etc/profile (sudo pico -w /etc/profile)
Relaunch terminal app and try it again. You might need to use it as root (via sudo), if it's installing stuff to system directories.
BTW... why do you need DarwinPorts to install Apache? Apache compiles just fine using the GCC that comes with Tiger (I know firsthand.) It's reaaaalllly easy to install via source in Tiger. (FWIW, my dev box runs Tiger, with Apache 2, latest PHP, MySQL, Postgres, etc... all built using the standard GCC4 that Tiger ships with in the dev tools package)
Regardless of how you get it installed...make sure you disable the binaries that come with the bundled version of Apache/Tiger; move or delete the htpasswd, htdigest, httpd, apachectl, phpize, etc... related stuff in /usr/bin, so you don't inadvertantly use the wrong versions.