Could you explain the meaning of warlock invocation number next to its grade?

I am rolling a warlock in my current campaign and we started at level 3. For my invocations, I have picked walk unseen and fell flight. I was told I could not do that by my DM since he told me my level wasn’t enough. However, when I referred back to my Complete Arcane, it mentioned:

A least invocation has a level equivalent of 1st or 2nd; a lesser, 3rd or 4th; a greater, 5th or 6th; and a dark invocation has a level equivalent of 6th or higher (maximum 9th).

So I automatically assumed that at level 1 and 2, I have access to least and 3 and 4, I have access to lesser, turns out not.

When I refered back to Complete Arcane for the invocations I picked, it said:

Fell Flight: Lesser; 3rd


Walk Unseen: Lesser; 2nd

I am really confused about which level can I pick these two invocations? I would appreciate it if you could explain how the warlock level corresponds to the invocation grade and the number next to it.