I have the same issue on EmeraldLakeBooks.com. Custom buttons are uploaded to https://emeraldlakebooks.com/wp-content/uploads/social/ and named to match the plugin’s buttons.
The path for the buttons is public_html/emeraldlakebooks.com/wp-content/uploads/social.
The plugin is enabled and the settings are all accurate. I even changed the button size in the widget to 64px when I realized that the icon set I got only had 64px images.
I can’t see anything else that’s potentially wrong…
Given the media changes to WordPress though and the fact that this plugin has not been updated to support WordPress 4.0 yet, and that the developer appears to no longer be supporting the product, perhaps I just need to find another plugin?