"I eat at six p.m."
It would only mean that if you replaced に with を.
It is very very commonplace to omit に in speech, particularly casual speech, though it is replaced with a slight pause (usually indicated by a 、when in writing).
「午後六時、食べます」 is an understandable, if somewhat emphatic, way of saying "I eat at 6pm" (although to me, it sounds more like "I will eat at 6pm").
The learning structure for this course is a little fast pace. Before it would introduce you to new words and test the hiragana of it on you, but now it just throws new words into ling sentences, (which the a.i doesnt say very well) and you have to guess or use the cheat ans click in the word. I started taking vocab notes on these words cause they dont explain them to you anymore, which inhope they fix. It should be apart of the lesson to learn am/pm and half past etc. I feel like clicking on the kanji is a cop-out. Id rather learn it first...
I can try f(^_^; bear in mind that both に and で have quite a few different usages, so I'll just talk about the basic ones you're likely getting confused by.
に is used to indicate the time at which something occurs. This is usually a "specific point in time" rather than a time period. (I used quotation marks there because it's not quite that clear cut, but it's a pretty decent approximation I think). For example, in this exercise:
「(私は)午後六時に食べます」 》 as for me (=私は), afternoon (=午後) 6:00 (=六時) is the point in time when (=に) I eat (=食べます)
We have also seen に be used to indicate the target location of a movement.
「(私は)学校に行きます」 》 as for me (=私は), I go (=行きます) to (=に) school (=学校)
As far as I can recall, we haven't yet encountered で in this course, but one of its roles is also to indicate location. However, で indicates the place at which something occurs. Here's an example of で being added to this exercise questions:
「(私は)家で午後六時に食べます」 》 as for me (=私は), home (=家) is the place (=で) and afternoon (=午後) 6:00 (=六時) is the point in time (=に) that I eat (=食べます)
All together, the above sentence means "I eat at home at 6pm." In English, we use "at" to indicate both time and place, but Japanese separates them into に and で respectively.
I... lived (and taught English) in Japan for two years, my partner is a native Japanese speaker. She's my resource f(^_^;
Starting out though, my main resources were anime theme songs, websites that hosted translations of the lyrics to those songs, time and enthusiasm for (language) puzzles.
I'm sorry this isn't as helpful as my other comment f(^_^;
This has been explained in earlier comments. Copied from a couple of my comments:
「に is used to indicate the time at which something occurs.」
「It is very very commonplace to omit に in speech, particularly casual speech, though it is replaced with a slight pause (usually indicated by a 、when in writing).」