You can easily add items to one of your Collections. Whenever you are using a calculator that you like and you want to add it to one of your collections, you can do it right from that page. You will see a row of buttons in the upper right corner of the page. Click on the 'More Actions' button. Under this you will see 'Add to Collection'. Clicking this option will open the Collection add box. You will see a dropdown that will display a list of all of your collections.
You simply select your desired Collection from the list and click add.
Alternatively you can create a new collection and add to it from this box by clicking 'start a new collection'.