First check that you installed rpy2 successfully. Look in /opt/local/var/macports/software for anything with rpy2 in the title. It maybe called py26-rpy depending on the version of Python you are running.
If you see that then you just need to use the right path to Python .. check the default location of Python like this:
which python
This will return the location of the first Python found and will probably say /usr/bin/python but you should use the version that rpy2 was compiled against.. which lives in /opt/local/bin.
Try:
/opt/local/bin/python2.6
then:
import rpy2