There’s a few differences between MODX Evolution and MODX Revolution. One of the biggest ones is probably that there’s no addons (like the GUI, or Rich Text Editor) pre-installed in Revolution.
Luckily, this is easy to get around by simple going to System in the top menu, and clicking on the Package Manager. From there, choose to "Download Extras" and there you can browse & search the MODX Extras Repository - which you can also find on modx.com/extras/ - and very easily download & install them.
For the GUI (Rich Text Editor), try searching for TinyMCE - it’s the same one that was included in Evolution by default. Download it, and then choose to install. It will automatically plugin to the content field and allow you to edit it just like you would in Evolution.
There’s more goodies to find in MODX Revolution, and perhaps the quick start tutorial could get you going:
http://modx.com/learn/quickstart/