Since FileMaker doesn't natively support HTML mail, I wouldn't expect it to have such an editing tool.
You can use a Webviewer. There you can load a html editor inside or just turn the webviewer into editing mode.
With MBS Plugins we include examples for this for use in FM Pro.
As ChristianSchmitz wrote: You can preview the html for your mail with the webviewer.
I expect that plugins for 14 should work fine with 15, but Dacon only mention 12-14 on their product page, I have asked them about 15 compatibility.
I use MBS to send my emails via the FileMaker server and it is quite reliable. There are several other good email plugins. But FM natively doesn't not support HTML email unfortunately. Personally, I use DreamWeaver to make my HTML emails and paste the text into a variable in FM for MBS to send out. Works for me.
You can email with HTML with our MBS Plugins too.
And we support FM 15 since last October, over 6 months before FM 15 was released.
does FMP 14 really not have a built-in HTML, visual or text editor to help users build calculations for a message in the Send Mail Options dialogue?
Tbh, while I can feel your pain, there are stronger priorities for a DBMS like Filemaker. Really.