They're named "custom profile fields
", not "custom fields". Which should pretty much explain why they aren't available in the editor.
Furthermore having categories and/or keywords is pretty pointless if you can't search for those - most users I've encountered who wanted to have additional fields always only thought about entering and displaying them, but never about how someone could filter posts/topics by just those new fields. Based on that and since posts are subject to the board search I can only suggest you're using the post text directly for entering your categories and/or keywords (that way you're halfway forced to use meaningful words and you can skip words which are already used in your post text).