How to download?

Since this post is under iPhone apps, this only applies to iOS. Also, your post is not clear. If you mean installing the current WhatsApp on older iOS, sometimes the current version of an app, not just WhatsApp, will not support an older iOS.

Whatsapp is a mobile messaging service, download for your phone either from Appstore or google play store. Not for a computer without a sim card as you need a valid number

As far as it goes, there are no current native desktop(read:windows) WhatsApp client. So if you still want it on your computer no matter what, then you have to do it the hard way. (via mobile emulator)
try this tutorial: http://techmell.net/buzz/install-whatsapp-messenger-desktop-pc-computer-easily/
just follow this through and you'll be set.

It is not currently possible to download WhatsApp Messenger for Mac. You can, however, download something called Bluestacks, which is an Android emulator, and it can run most Android apps on a mac, and you can then run WhatsApp in Bluestacks. Visit bluestacks.com to download.

download and install blackberry pc siute latest version, connect yo fone. google whatsapp for blackberry n download the installation file then use the blackberry pc suit applications panel to browse to the file n install onto phone

the new whatsapp app is not running on a iPhone 3G with iOS 4.2.1 as it is too old...
- but there is a workaround which you may try:
It is possible, a bit tricky, but possible. First, you'll need to download the Whatsapp app through iTunes on your computer, this will make it show up as "purchased" in your library.
After that, open up the App Store on your iPhone and go to the 'purchased' tab. When you try to install it this way it will tell you that you need iOS 6 or higher, but will also ask if you would like to download an older version of the app. then click on download.
- again step by step:
1) Open up iTunes on your computer
2) Go to Store (left hand column) ->iTunes Store
3) Search for Whatsapp; - download the app
4) then open App Store on your iPhone
5) Click on 'Purchased'
- you will have to install Whatsapp by clicking on it as usual.
App Store will ask you: "Download an older version of this app. The current version requires iOS 6.0 or later, but you can download the last compatible version.",
- then click on Download and it should finally work.

It is possible, a bit tricky, but possible. First, you'll need to download the Whatsapp app through iTunes on your computer, this will make it show up as "purchased" in your library.
After that, open up the App Store on your iPhone and go to the 'purchased' tab. When you try to install it this way it will tell you that you need iOS 6 or higher, but will also ask if you would like to download an older version of the app. then click on download.
- again step by step:
1) Open up iTunes on your computer
2) Go to Store (left hand column) ->iTunes Store
3) Search for Whatsapp; - download the app
4) then open App Store on your iPhone
5) Click on 'Purchased'
- you will have to install Whatsapp by clicking on it as usual.
App Store will ask you: "Download an older version of this app. The current version requires iOS 6.0 or later, but you can download the last compatible version.",
- then click on Download and it should finally work.