In what instances would a Lithuanian use the word pasiilgti in the present tense? Or is it always used in past tense, if so, how does one differentiate between the English sentences

I missed you (said to someone you meet after a long time)
I miss you (written to a loved one far away, hoping to meet them)

Google is of course not a good guide but the present tense seems to be used also.

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"...kur daug patarėjų, - pasiseka."

When you say "I miss you" you and use past form in Lithuanian (aš tavęs pasiilgau), it slightly denotes that you've been missing that person more and more every day, that feeling has been getting stronger and stronger. And present form (aš tavęs pasiilgstu) does not emphasize that feeling so much. But this difference is very slight, you can use whichever form you want.

Then I should apologize for my correction here:
http://www.debeselis.net/forum/viewtopic.php?t=815

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But what do I know? I'm just an Argentinian.

No, you shouldn't, everything is alright over there.

I don't care about who's right or wrong, I just want to understand the language. I'm a Finn, so my default value is that I'm always wrong. But I do find the information contradictory...

"But this difference is very slight, you can use whichever form you want."

So:

I miss you = aš tavęs pasiilgau (or aš tavęs pasiilgstu), but preferably the first choice

I missed you = aš tavęs pasiilgau

Then this comment "It has to be used in the past form" can't be all true. So "everything is alright over there" can't be all true either.

So saying "aš tavęs pasiilgau", although the missing still continues a Lithuanian is emphasizing that the feeling has been and is getting stronger. This is very interesting, from my language background it's hard to understand.

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"...kur daug patarėjų, - pasiseka."