In chapter 2, section "gismu as sumti":
...you don't need cu after mi (I/me/we), do (you, the person(s) I'm talking to) or any words like this ('pro-sumti', in Lojban jargon).
In chapter 3, section "Questions":
...mi cu na nelci la bil. means "It is not true that I like Bill," or in other words, "I don't like Bill."
Tip: By default, na is followed by a selbri. Since cu has the job of indicating that a selbri is coming up, na makes it superfluous. So you can simply say mi na nelci la bil.
The use of cu after mi in the second paste is not explained, and to say that it is not needed because the na is there is to say that it would be needed if na were not there.