I have not found a way to create a custom place by dropping a pin, but have found an alternate way.
1. Mark the photo(s) you want to add the location to.
2. Press the "info"-button to open the sidebar at the right side of the screen.
It should be a little map at the bottom right. If not, press the little bar, and the map shows.
3. Find the coordinates for the place you want to add.
It can be done several ways, but an easy one is to go to http://maps.google.com/maps and find the place you are looking for. Then right-click on the place and choose "Directions from here". The coordinates show up at the right side of the map.
4. Copy them.
5. Insert them above the map in iPhoto, and choose the Google-option.
6. After the little pin have marked the place you just entered, mark the coordinates and type in the name you want it to have. Choose the option that changes the name of the location.
Hope it helps .
as a practical matter, choose a place for the photo(s) from some place near-by.
then follow support document to rename the place, and in the litte iphoto places map, drag the pin.