I've been surprised at the number of people who keep asking about how to buy DAO tokens so I thought it might be useful to start a new topic about it.  If you are not technically minded or have not recently updated your MIST wallet and don't want to wait for it to update the Bittrex DAO page is a quick and easy way to do it.
Before starting you will need to have some ETHER.  ETHER is the currency used to purchase DAO.  The easiest way to get it is in exchange for Bitcoins and you can do this directly on any exchange (including Bittrex).  At the initial price you get 100 DAO tokens for 1 ETHER (https://daohub.org).

A) Transfer Ether to your Bittrex wallet (if it is on another exchange or in your MIST wallet).  If you don't have a bittrex account you will need to make one first. (https://bittrex.com)

B) Once the Ether is in your Bittrex Ether wallet you can go to the Bittrex DAO page (https://bittrex.com/balance/dao).

C) Click on the generate address button to generate your DAO address (Step 1) then enter the amount and click on the transfer button (step 2).

D) Confirm you want to withdraw your Ether to the DAO creation address (enter your 2fa code if required) and wait up to 30 minutes.
 
E) After a while the number of DAO tokens will be listed on the same page under where it says Step 3 "Validate your balance".


Important Notes:

-If you are really wary or unsure try going through the process with a small number of Ether.

-You can do it as many times as you like during the DAO creation period using the same address. Just remember to go through the steps as above (except for Step 1 on the DAO generation page as you only need to do this once).

-You will always need to send your ether to the regular Bittrex wallet first then use the Bittrex DAO page to transfer it. You can't just send directly to the DAO creation address.

-You won't be able to trade DAO or move it from Bittrex until the DAO creation period is over (28th May 2016 at the earliest).

-For more information on the dao go to: https://daohub.org

in announcement DAO interest causes the market of activation of project...

I agree you can find the latest versions of MIST here:

https://github.com/ethereum/mist/releases

It is definately the best ethereum wallet.

It takes a few hours. You still have circa 21 days to jump on the DAO.

OP, where is the generate button?

When will the DAO tokens be tradable do you know please?

It's not in your wallet.  It will be on the same page where you create the DAO tokens and you need to wait some time before it will appear (where it says "Step 3 Validate Your Balance").  The tokens will only be available to trade after the DAO creation period (28th May at the earliest).  If you still think there has been some sort of error submit a support ticket to Bittrex and they will be able to help you out.

They are tokens for a Decentralised Autonomous Organisation and grant the holder certain rights within the DAO entity. It's like a new company where the token holders get to choose what it invests in and get rewarded as part of the investment.  It's similar in some respects to a venture capital company but there has never strictly been anything else exactly like it before.

You can read more here:

https://daohub.org

There is also a useful discussion on episode 57 of The Bitcoin Podcast which explains some of the basic concepts quite well:

http://thebitcoinpodcast.com/episode-57/

It's obvious you don't understand how the DAO works so why did you want to buy it in the first place?  You definitely should not be investing if you can't even be bothered to find out what you are investing in.

DAO is based on ether and the ethereum blockchain so obviously it has to be created using ether. 

I don't see why this would be an obstacle to anyone.  If you don't have ether, buy some ether.  You can buy it on Bittrex or any of the other exchanges where it is traded. 

If you can't be bothered with a regular exchange Shapeshift (https://shapeshift.io/) makes it even more straightforward. 

You would have to be monumentally lazy to pass up the opportunity of making money just because you can't be bothered to exchange some BTC to ether first.

Thanks for the link.