After a user installs CloudExtend Excel for Salesforce the CloudExtend icon will appear on the data tab in Excel. From time to time the add-in may disappear from the data tab.
If this is the case first check Excel to make sure you are logged in to the same Microsoft account you installed the add-in from. If you're still not seeing it some action taken on the Excel side may have removed it (or your data tab ran out of room). See the GIF below to learn how to get it back.
Click File->Options->Customize Ribbon
Look for CloudExtend in the left most column and add it to the data tab in the right column.