Your wallet program doesn't show the private keys because you usually don't need to see them. In the same way that you almost never see your password to any website you use. If it did show them then someone could take a photograph of them and later spend the content of your wallet.
It probably doesn't let you delete addresses because of the potential for harm. If you ever gave that address to somebody and they some day transfer coins to the address, you'll wish you hadn't deleted it. If you send someone an address and they use it to pay you, and then you think "that transaction's completed, I'll delete the address now", you'll lose the coins that were sent to you because they're still sitting in that address. There's too much risk of people not understanding the system and losing coins as a result.
To send money from one computer to another:
- first go to the computer that's going to receive the money, run the wallet program, go to the 'receive coins' tab, pick any of the addresses that are shown, optionally edit the 'label' field to make a note of who you're going to give it to. In this case "My Mac" might be a good label. Then copy the address to the clipboard and email it to yourself at the Mac. Or write it down if you like.
- then go to the Mac, run the wallet program, go to the 'send coins' tab, put the address from the previous step in the 'pay to' field, and the amount in the 'amount' field. Optionally put a label in the 'label' field. "My PC" might be a good label for this. Click "Send".
I just noticed you want to send from a specific account on the Mac. Do you mean you want to send from a specific address? That's not yet supported in the standard client. A patch has been submitted that adds this feature. There's a description and video going into more detail.
Edit: it occurs to me that you really to mean "account" in your question. Don't confuse accounts with addresses. You don't need to send from a specific address to send from a particular account. There's no support for accounts in the bitcoin-qt client though I don't think.
The upcoming 0.6.0 version of the Satoshi client adds a command to export an address' private key, so if you want to print it out and keep it somewhere safe you could use that. But typically people just make sure they have a copy of their whole wallet.dat file instead.