How to represent GND pin in CC2640 VQFN 48 pin package?
Many ICs have an exposed pad underneath.  Sometimes is for grounding, sometimes for heat dissipation, sometimes simply for mechanical support.  I've seen one (an IMU) where the exposed pad was supposed to be left unsoldered.  The datasheet usually makes it pretty clear.
Your specific part's datasheet, however, isn't so clear.  In addition, this part has different grounding methods for each of their different packages.
For the VQFN-48 (7x7mm) and the VQFN-32 (5x5mm) packages, the only ground connection is the exposed pad.  But, for the VQFN-32 (4x4mm) package, there are actual Vss pins in addition to the exposed pad.
These signals can be seen in Tables 4-1 through 4-3 of the linked datasheet, but it is confusing that the extra Vss signals would simply seem to be omitted.  However, if you look at the example circuits in Figures 7-3 and 7-5, you can verify the intention of the datasheet.
Unfortunately, they don't have an example circuit for the VQFN-48, but comparing the VQFN-32 tables with the VQFN-32 diagrams implies that you can trust the VQFN-48 table.
(note that each image has a Vss pin labelled &quot;33&quot; in a 32-pin package)
Figure 7-3: 5x5mm VQFN-32:

Figure 7-5: 4x4mm VQFN-32:


As far as MultiSim goes, there is an answer to your question here at the NI Forum:

Generally thermal pads are added using a the rectangular SMD pin (or custom pad shape if you require a cutout). an example using the custom pad shape already in the database is the QFN40P600X600X80-49N.
...

You should go there to read the whole answer.