lastname,firstname. Furthermore, you must not enter more than 15 characters (The form blank will accept only that many and will "beep" at you if you try to enter more). In general, you need to enter the last name and enough of the first name to guarantee uniqueness.
ripken would give you Cal and Billy.ripken,c would be enough to get just Cal.williams would give you a bunch of guys.williams,g would narrow it down but still give you more than you wanted.williams,ger would probably be enough to give you just Gerald.grudzielanek,ma.grudzielanek,mar which exceeds the 15 character limit.There are probably a few guys that share the same names (at least in different years) and in those cases you'll just have to be aware that you're getting data on more than one person.
If you think you're entering a name correctly but you're not getting the expected results, you should probably go to Doug's site and see how the guy is listed there.