Click on "My Device & Media>Messaging" Or you can bookmark this url; http://sprint.zipwhip.com/ Or if it is another Sprint user you can send it from your email by addressing it to firstname.lastname@example.org Other carriers also have messaging from your email to phone address's you can google for those if you want.
Here's a site I had bookmarked, note that the thread is over 3 years old, so things my have changed:
Amp'd Mobile @vtext.com
Verizon @vtext.com (text)
Verizon @vzwpix.com (pictures)
US Cellular @email.uscc.net (text)
US Cellular @mms.uscc.net (pictures)
Cingular @cingularme.com (not sure)
Boost Mobile @myboostmobile.com
Einstein PCS @einsteinmms.com
But what if you're not a Sprint customer but need to send a text from your computer to a Sprint customer? I'd hate to have to walk down 4 flights of stairs and outside my building to get a signal every time I want to send a text to a Sprint customer while I'm at work. Verizon & AT&T let you send texts from their websites, so why doesn't Sprint? I can get the spam issue, but if you create a login for Sprint with your email account, there's still no way to do this -- unless if you're a Sprint customer. It's quite ridiculous if you ask me.
But what if you're not a Sprint customer but need to send a text from your computer to a Sprint customer?
You can send it from your email by addressing it to email@example.com