I don't think the plugin shortcut button is being automatically added to the plugin toolbar. To check if it is added:
Edit Book|Preferences...|Toolbars|Book wide tools from third party plugins
Make sure the plugin icon "文" is in the right side "Current Actions" panel.
If it is, try removing it and then adding it back in.
Also make sure the plugin toolbar is being displayed:
View|Toolbars|Plugins tool bar
There is an open Github issue on this for the plugin:
https://github.com/Hopkins1/TradSimpChinese/issues/25