I've never used the mac version, but assuming it is similar to the PC version, you should be able to simply click the receive coins tab. The "address" column will contain the strings you will want to enter into the watch-only field on Blockchain.info.
OK, but these are the addresses that you can create for each incoming transaction to determine who the sender is. I can only enter one public key into the "Add Watch Only Address" field in blockchain.info. Are you saying that I can pick any address and I should be able to watch the account?
Actually, I don't think the watch-only address feature is very useful because blockchain.info makes no distinction between the spendable amounts and the watch-only amounts. I added some paper wallets as watch-only and now it looks like my wallet has a lot more than it actually has. If I wanted to use the watch-only feature for keeping track of paper wallets, I would set up a separate wallet for that.
Anyway luong, I think you still don't quite understand. A wallet is a collection of addresses, some that you create and others that the wallet creates for you automatically.
Here's a example scenario:
1. You create an address in your Bitcoin-qt wallet for receiving payments, e.g. 1xyz....
2. You add 1xyz... as a watch-only address in the Blockchain.info wallet.
3. Someone sends 10 BTC to 1xyz... and your Blockchain.info wallet shows an value of 10 BTC.
4. You send 3 BTC from your Bitcoin-qt wallet to someone at 1abc... The wallet sends 3 BTC to 1abc...
and 7 BTC to a new hidden address. Meanwhile, 1xyz... is left with 0 BTC.
5. Even though your Bitcoin-qt wallet still has 7 BTC, the Blockchain.info wallet says you have 0 BTC because it doesn't know about the hidden addresses.
Here is another scenario:
1. You have a wallet with 1 BTC and many addresses.
2. You create a new address in your Bitcoin-qt wallet for receiving payments, e.g. 1xyz....
3. You add 1xyz... as a watch-only address in the Blockchain.info wallet. The Blockchain.info wallet shows a balance of 0 because it doesn't know about the other addresses in the wallet
This is why the watch-only feature is useful only for addresses in wallets that are normally inactive, like a paper wallet or a brain wallet.