If you click on the add-on on the left pane and then click 'install' you should then get a pop up which states that this has been installed.
If you are accessing the fermentables list within a recipe, make sure that the box at the top labeled 'show only items in inventory' is not checked.
I have seen this posted quite a bit lately on other forums and not sure if it is one or a few people having an issue or if there is something blocking their version of BeerSmith from accessing the add-ons. I have been able to install and uninstall several add-ons successfully the past couple of days to test it and have had no troubles with the new items showing up. Given this, I suspect it is either the setting referenced above or a local machine issue.