If you find that a model is being used that lacks a particular Data Element, then you can import one from another Data Model, or alternatively you can create a new Data Element.
Step-By-Step Guide
First make sure you have selected the Data Model from the Dashboard that you want to work with.
Select the "New Data Element" button on the menu bar at the top of the screen.
This should bring up a "Wizard" dialog box in the middle of the screen - simply fill in the details, name and description and the select an existing datatypes or create a new one for this data element. 
When you come to datatype, you may choose an existing one (via search) or you may create a new datatype.
If you select an existing datatype which is not currently in the model you will get 2 options: import or clone. In effect the clone will copy the datatype selected, and build an exact copy in the current model. Import will import a reference to the datatype, this datatype cannot therefore be changed in this model, in fact most imported datatypes will be imported from a finalised and therefore unchanging source dataset, such as XML Schema.
In this example we create the first datatype for our data element "Abative Therapy" as a cloned xs:String, which means it is an exact copy of the XML Schema String datatype. It can however be changed locally, and this will not affect the way the xs:string is defined by XML Schema.
In the screen on the right the tag next to xs:string is labelled in the first case in yellow, to indicate that is a datatype belonging to this "DRAFT" datamodel.
We have also created a data element called Adrenal invasion, again we want it to be a string datatype, but this time we import the data element from the XML Schema model. It is is being imported from a finalised and therefore fixed model, and this cannot be changed locally.
In the screen on the right in this "imported" datatype case the tag next to xs:string is labelled in the first case in blue, to indicate that is a datatype belonging to this FINALIZED datamodel.
So although the datatypes are both called "xs:string" they are in effect different datatypes.