Login to WIX
Select the website you have your Blog on
Once selected, on the left hand side you'll notice "Wix Blog (NEW)
Click on it and you should see on the right hand side "Create a Post"
Let me try my best to help you get started with the New WIX Blog. First if you haven't already created a website you'll need to do so by simply clicking on any of the templates or do it yourself at www.wix.com
Open your editor and on the left hand side you'll notice a Pen Icon - You can get started there.
Now if you already have a website and would like to get started Blogging make sure again you've added a Blog to your website. Again within the Editor on the left hand side you'll notice a Pen, Click it and get your Blog Started.
If you have already added a Blog the best way to write a Blog Post would be simply starting from your WIX Dashboard.
I hope this helps?