Welcome to the forum, Gregory.
Without an sample (some lines of code) of what is wrong, it is difficult to help you.
You do not have problem with MESSAGE but you have problem with MESSAGE_INFORMATION_OK.
Maybe you did not see that MESSAGE is a command, so you use it well:
MESSAGE "Hello"
but that MESSAGE_INFORMATION_OK is a function which returns a value.
So the use is like that:
if MESSAGE_INFORMATION_OK("Hello")=1 then - - -
But you are right, the reference manual is not accurate about that and I will correct it: the given examples are bad.