When trying to add a new field to a shapefile in ArcCatalog, I have been told to type the field name manually into the vacant column in the shapefile Properties> Fields tab. However, the system won't let me type in the row under the field name column. I type and nothing shows up.
At the suggestion of Chris W I'll post the comments as an answer in case it helps others in the future.
Make sure that there are no other connections to the shapefile you are trying to change. Schema changes require and exclusive lock, which means no one except you can be looking at the shapefile and even then you must only be looking at the shapefile from one application. If you have both ArcMap and ArcCatlaog open and pointing to the shapefile this might prevent an exclusive lock.
Another approach is to try to add a filed through the table properties drop down. open the table part or the shapefile (look for a dropdown under the preview tab that says geography and change it to table) and then look for table options dropdown. From here there is an option to add a new field. The advantage of doing it this way is if there are existing locks it will tell you the names of the user creating the lock.
One other possible solution is Mapperz suggestion of ensuring that the shapefile is no read only.