Sensors with direct integrations will only display in the dropdown once they are plugged in. Once plugged in, IRIS should detect the new senso and automatically update the page with your sensor selected. *Remember, when capturing images Step 1 is to plug in your sensor first and confirm you see the correct sensor name.
Try restarting image capture service as described here.
If you are using a brand new sensor, your configuration settings file may need to be adjusted. In this case, please reach out to IRIS support.
If you have a Twain configuration (not a direct integration), and can not find the pop up box to click on. Check on the desktop menu bar. The pop up may be hiding behind your current window you are viewing. Click on the icon to bring forward.
