Hey guys, you know the part when the Don and Sollozzo meet at Genco imports, well theres a part where the Don asks Sollozzo "what about the Tatagglia intererst?" And Sollozzo then turns to Hagen and says "My compliments"...Why does Sollozzo say this to Hagen? What does it mean? I still don't understand it

Thanx

It mean that he applauds Tom for finding these things out.

Possibly also congratulating Tom for being named consigliere, he had replaced the terminally ill Genco as consigliere fairly recently at this point.

Tom arranged the meeting. We don't know exactly how, or who contacted who, but you will recall that after the horsehead scene Tom is telling Vito about the meeting and over the narrative we see Sollozzo climbing the stairs inside Genco.
As consigliere Tom had to have as much background on Sollozzo as possible. Remember Vito's lesson, keep your friends close, but your enemies closer. Sollozzo was acknowledging Tom's background check and acknowledging that it was true. He was looking for Vito's support, so hiding things in the meeting would not help to create the alliance Sollozzo was looking for. But as we know Vito nixed the plan.

Yes, that is true. Tom was German Irish, and as such, many of the other 5 families did not give the Corleone family any credit for Tom's knowledge and finding out minute details which would be useful to the Godfather. (This is mentioned much in the book.) Tattaglia gave his compliments showing that he respected Tom and his effort in finding out that information.

I inferred that Sollozzo hadn't volunteered that he was giving a share to the Tattaglias. But he should have known that Vito and Tom would figure that the Tattaglias would have to get something from Sollozzo in return for being his partners. (In the novel, Hagen tells Vito that Sollozzo "is vouched for by the Tattaglias.) Maybe Sollozzo was just buttering up Tom.

In the Novel Sollozzo seemed nervous when Vito asked the percentages of the Tattaglia Family. Why do you think he was nervous?

In the scene in the novel just prior to the Sollozzo meeting, when Hagen is briefing Don C., he says the Tattaglia family "May have a piece of the action". (italics mine)

When he gives Don C. his reasons why he should accept the upcoming Sollozzo proposition, he is looking to the future and the enormous profits to be derived from the drug trade, which, if realized by one of the other families, could make them more powerful than the Corleones, and he tells Vito

"If we don't get into it, someone else will, maybe the Tattaglia family" (italics mine)

Then, at the meeting itself, when Don C. asks Sollozzo "What is the percentage for the Tattaglia family?", there is no "My compliments" comment from Sollozzo to Hagen.

So, it would appear that while the Corleones were not 100% certain that the Tattaglias were involved, Don C. took a shot that they were with his question to Sollozzo, and the question hit home.

Sollozzo probably figured that his deal with Tattaglia (and Barzini) was secret, and he became nervous that Don C. was one step ahead of him.