To add any ingredient to your inventory, you must first add it to the appropriate database. If you look through the 'Misc' items, you will already see a number of fruit purees listed.
To add an item, open up the ingredient database to which you want to add it. For example, let us add a 'mango puree' to the 'misc' ingredients file. So from the BeerSmith page you are on, click on 'ingredients' on the top menu, then select 'miscellaneous'. This will open up the ingredient database for all materials classified as a miscellaneous ingredient. On the ribbon above the tab will be an option to "add misc'. Click on this and you will get a misc item tab opened up. You can enter as much of the information you want/need/have regarding the ingredient. In my example, I would enter the name as 'Mango puree' and change the 'type' on the drop down menu to 'flavor'. Now you can change the amount you want to add to a typical batch size and the select the units of measure from the drop down menu. In my example, I would add '500' to the amount and change the units to grams ('g'). Now I would change the batch size to my normal recipe size of 10 liters. If I already have the puree, I can then enter the amount I have on hand as the 'inventory' and the price per gram of the puree. You can then adjust the use for where you want it to be typically added. So in my example, I would change the 'use in' to 'secondary' and add the 'time' as '5' while changing the units to 'days'.
Click on 'ok' to save and exit.
You can do this to add any ingredient: Grain, yeast, hops, or miscellaneous to the appropriate database. It will then be available to add to your recipes and keep in inventory.