Are you asking about how to chat and text message on your Yahoo! Messenger or on Yahoo! Mail? There is different ways to do all that.
You can do both with your Yahoo! Messenger and with you Yahoo! Mail account.
When you sign into your Yahoo! Messenger then you have to click on the contact in your contact list to open up the chat window. Then you just type your message into the box on the bottom and then click on "send" on the right or just click on "Enter" on your keyboard. Your message will show in the top box and your friend can see it and respond to you.
If you would like to send that contact a sms then you can click in the chat window or on the Yahoo! Messenger itself on the top on " Action" then on " Send an sms Message " and this will open up a little separate window. If you have the contacts mobile already stored in your address book then you just click on the contact you wish and if not then you click on top on the option " other contacts" and there you just type in your friends phone number. You have to put the phone number in the right format though. There are 3 examples for you right under the box where you have to put the phone number in. For the US for instance you have to type it like this +1-123-123-1234 ( country code + area code + number of person ). You can type it then like this also +11231231234
Once you have done all this just click on ok and it will open up the sms window for you. Just type in your message on the bottom then click send or enter and your message will be send to your friends mobile phone. Keep in mind that your friend will have to respond to your first sms in order for you to be able to send more sms's to that same number. Yahoo! does that to protect people from spamers. You will receive your friends sms on your window on your screen and you can sms back an forward like that.
If you have already put your friends in your address book with their cell phone numbers then you just need to click on the contact on your address book and in the little window that opens click on the little sms mobile sign to the top right...It will open up the sms window also.
Now, you can do the same thing with your Yahoo! Mail account since quite a while. Let's say you are on a different computer that doesn't have Yahoo! Messener downloaded or if you are in an Internet shop using a computer there and they don't allow you to download the Yahoo! Messenger then you can just logg onto your regular Yahoo! Mail account and send sms from there or send IM's and chat with your friends from there too. Here is a copie of one of my answers to that :
" Do you know that you can actually do your instant messaging chats also from your Yahoo! Mail account and you do not need to download the Yahoo! Messenger for that any longer. You can also send free sms to your contacts from there too.
You just go to the regular Yahoo! Mail https://login.yahoo.com/config/mail?.intl=us
and log into your account. Make sure you do not have the Yahoo! Mail Classic but the new Yahoo! Mail turned on. You can switch between those two back and forward. With the new Yahoo! Mail you just click on the top left on "New" and then on "Chat" and then in the little window that opens up you click on "Sign in to chat". It will sign you into your chat and you can choose your contacts on the left and type in your IM's on the bottom etc... This way you will not have to download Yahoo! Messenger but you still can chat with all of your contacts and you will also be able to send them sms on their cell phones and receive sms back in your chat window. So have fun exploring!

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