There are two possible causes for this: either you do not have write permission to the file or you do not have a license.
Cadence will allow you to open files in read-only mode without a license, but not in edit mode.
Try typing env at a unix prompt to make sure you have your license path set up correctly. After LM_LICENSE_FILE it should list /usr/site/cadence/licenses/license.ferr (if it has other paths as well, that's fine)
If that doesn't work, check to make sure that you have write permission on the file. To do this, type ls -al /home/user_name/your_cadence_directory/library_name/cell_name
The files permissions for this directory should be drwxr-xr-x, the single w signifies that only the owner of the file has write permission. If you are not the owner of the file, you will not be able to make any changes to the file.
This will happen if you don't create your library correctly. Try following the directions under Library more carefully, copy the directory containing your design into the new library, and try it again.
For more information on this see http://www.cadence.ncsu.edu/doc/cdsuser/verification.html#dio_layer