Marris Otter is simply English 2 Row Pale Ale Malt.
For Crystal what we have in the stock database is 10,20, 30, 40, 55, 60, 75, 80, 90, 105 and 150 L varieties.
You can do a couple of things to get your entries in. Here's what I would do for Crystal 120:
1) Go into the grain database, select (highlight) crystal 150.
2) Now on the right side see the "Clone" button? Press that. You will immediatly see a second Crystal 150 entry.
3) Now double click on the new entry (or use the Edit button) and change the name to Crystal 120, and the color (SRM) to 120.
4) Save the database when finished.
You could have also just added the Crystal 120 entry from scratch. ProMash lets you add, modify, delete or clone entries into any of its databases so if an ingredient is missing you can simply add it yourself.
Hope that helps,