First don’t enter unknown (or an abrieviation) or a “?” when you don’t have the first name or surname. I’ve seen trees online with a surname of “unk” or “mnu” (maiden name unknown) and then people copy that and think it’s really a name. So how do you let the software know that you don’t know?
If you know the given name but not the surname, you should enter two backslashes (the key that is usually above the “enter” key on a PC) in place of the surname. This will make the given name show up properly in reports and trees as a given name instead of as a surname. This is very handy for many great great grandmothers in my tree. So Ann Unknown should be entered as Ann\\ and Native American Sitting Bull becomes \Sitting Bull\ \\.
The same backslash symbols are also used when the surname has a space or is two words. This keeps the pieces of the surname together without having to remember arcane keyboard combinations like “ALT+0160” for a non-breaking space. This means that “Van Wagner” could be entered in Family Tree Maker as \Van Wagner\ instead of VanWagner. This may work in other software but I’ve not tried it.
See my blog entry “We may need a little space” for other hints regarding names with and without spaces.