For custom fields i recommend you map them in Preferences/ Tags(1)/ custom tags so any updates done in MB are saved
Search the forum for posts from other mediamonkey users who have already done this
Play count and date added can be imported from an iTunes xml file, for which there is a MediaMonkey plugin that generates that file.
Again search the forum as it has been discussed before
if you still want to use the API, date added is 12 and play count is 14. ie. cast those values to MetaDataType and use Library_SetFileTag
and MB_RefreshPanels once at the end