Choose "Save to Gallery" as png and then touch the project you want to save. After that the OK button gets enabled. see also my attached screesnshot
Hope that helps.
In addition to greule's helpful post, I have posted several screenshots to illustrate this process.
1. As you can see below, Tap the Share button, then Save to Camera Roll (you can also select upload to creative cloud, or share, etc). For this demonstration, I have selected the "Save to Camera Roll".
2. The Save to Camera Roll dialog will appear, prompting you to select the photo or photos you would like to save. Select your photo(s).
3. Once photo(s) are selected, select the file type you would like to save. In this case, PNG.
4. When done, Tap OK.
I hope this helped you, please post back with any questions. In addition there is a helpful tutorial on how to save for Facebook, or Photoshop.