You need to supply which of the union members you are addressing. The compiler needs to know if you want to test ull
(as unsigned long long) or sc[0]
(as unsigned char), or any of the other members.
In your case you probably want to use
if(increasingMessage.cd.us[3] + 1 > 65535) {
...
-- that is, if this part of the union is the number you are looking for. (It's not entirely clear from your code.)
Union members are accessed the same as struct members, they only difference is that they get stored differently.
Note that the particular field I chose will never be >65535 ... So choose your test member carefully ...