My first simple sentence

[Niawve Mokri]

Krr a nga z<am>a'u, fitseng oe l<am>u.

When you came, I was here.

If there are any mistakes, please let me know. 

[NeotrekkerZ]

To be at a place/location is tok not lu and it takes the ergative/accusative:  oel tamok fìtseng-it.

Also, the beauty of the krr a/a krr phrasing is you can move entire clauses around.  So if you didn't want the comma:

Fìtseng-it oel t<am>ok krr a nga z<am>a'u. would mean the same thing.

[Niawve Mokri]

Interesting...Irayo!

I'll try to remember that! 

[Carborundum]

Another neat feature in Na'vi is that tense, aspect and mood can be omitted once determined. In other words, you only need the <am> infix in the first verb, it is then understood in every following verb.
So Fìtseng-it oel t<am>ok krr a nga za'u would also work 

[kewnya txamew'itan]

In this case, whilst the first clause (when you came) is correct, using <ol> is a bit more appropriate.

Knowing when to use <ol> and when to use <am> is tricky, but in this case, as you are referring to them comming as being a single point in time (which you've done here with the krr a), it is definitely a case whee <ol> should be used.

In the second clause, <am> is correct as that action is not necessarily one point, nor is it necessarily ongoing so a past imperfective would be wrong.

So actually a corrected version would be:

Fìtseng-it oe-l t<am>ok krr a nga z<ol>a'u

[Niawve Mokri]

These are all great tips. Irayo! So if I were to correct the sentence, it would be sorta switching the phrases around so that it wouldn't be Krr a nga za'u fitseng-it oe t<am>ok, it would be Fitsent-it oe t<am>ok krr a nga za'u?

[kewnya txamew'itan]

you can switch the clauses around provided you try to keep it consistent.

That said, if you're using <am> not <ol> for the za'u (which is OK(ish) but <ol> is better) then <am> goes in the first verb so it would go in the za'u in you new sentences.

You're also missing an ergative marker on oe.

So what you'd get is:

Krr a nga z<am>a'u fìtseng-it oe-l tok


But the following is better because, as I said in my first ost, <ol> is more appropriate in the za'u:

Krr a nga z<ol>a'u fìtseng-it oe-l t<am>ok.

[Niawve Mokri]

I'll have to watch out for that ergative marker.