I just encountered a bug in interim release MP4-v4.623.
As administrator in the Campaign Reinforcements section of Campaign Management, when I select an air supply point, an available aircraft, specify the number and press the arrow (which is below rather than adjacent to the data line) I get a pop-up window that says:
Unfortunately I am at v4.695 now, and I cannot easily roll things back to your revision to test. As a last guess, if you are using Internet Explorer 8 or Firefox witth IETab2, make sure you turn Compatibility Mode ON in IE8 and then restart all your browsers.
If this fails, as a work-around, just add the types you want to you Resupply_Replacements table directly. For ObjType, use the Object_Specifications.Object_Type value for the object you want. Leave Committed and Used fields as zero.