Hi TOSHI and Gabriella,
I did what you said (been doing that actually) but unfortunately there is no icon ("'Add NextGEN Gallery' icon") that pops up.
HOWEVER, I scoured the internet and found this link in NEXT-GEN's site:
Find the shortcode of the style of slideshow or gallery you like then copy and paste into your post/page in the edit mode. Make sure you paste the shortcode into the Visual tab.
An ex. of a shortcode listed there is :
"[ nggallery id=X ] without space before/after brackets."
So copy/paste this ---> [nggallery id=X] into your Visual tab. X is the id no. of the gallery you created. You can find it in Gallery/Manage Galleries. Click Update, then View.
I am finding the NEXT-GEN GALLERY plug-in behaves differently in different platforms. I am using CHROME and MINIMATICA THEME. So good luck. This works for me in the absence of that "INSERT" button. Good luck, Gabriella.
Thanks for your input though Toshi!