To clarify, in the admin area for widgets, you have placed the widget in the sidebar area, and you can change settings for it, save it, but the widget won’t show on the front end?
Try turning all plugins off, then turn on the plugin. If you can add your widget and it work, then turn on the other plugins one at a time, testing to make sure the widget is still working. Eventually, you’ll probably turn one on and the widget will stop working and you can let both plugins authors know that their plugins don’t work well together.
If with all other plugins off, you can’t get the widget to work, then switch to the twentyeleven theme and try again. If the widget works now, run through the same steps as above. If it does not work, then the plugin itself is to blame.
Most of the time when things like this happen, it’s a social sharing plugin or something that has outdated code and is interfering with your widget, but it could be that the plugin author has released a version of the plugin that simply doesn’t work with your host (less likely but possible).