1. I think you mean, "I regretted you couldn't come." This means they felt bad that the other person couldn't come.
2. This one is hard because I don't know the story. I think one character is telling the other character that he must to do something big and obvious or the boy wont understand. I need to know more about the story to answer this.
3. "Mess about" is like "play." The character is saying, "If you want to help him that is OK. But make sure you are serious about it. Don't just play games"
4. I think you mean "bugging me." A bug is an insect, like a fly or bee. So "don't bug me," and "bug off," mean he is being like a fly or bee or mosquito and the other guy wants him to go away.
5. Again, I'm not sure because I don't know the story. The person saying this is trying to be serious, but the other person thinks it is a joke. So the first person is asking him to be serious.
6. This is something we say a lot. It means, "maybe you don't like what I have but it is what I am offering (giving) to you. It is your choice. You can take what I have or not."