Hi,
There are a few resources that will be helpful for you.
Because this will be a rectangular block diagram to represent the component, you'll be probably best to follow the Component Wizard (Tools > Component Wizard). This will walk you through the process of creating the component, which you can make according to the block diagram in the datasheet.
There's a tutorial here:
Creating a Custom Component in NI Multisim - Developer Zone - National Instruments
Also, there's a YouTube video that shows another method of creating a component that may also be helpful/supplemental.
Regardless of the methods that you choose, you will need to edit the symbol and rename pins to match the pins in the datasheet. Because there are a couple I/O arrays (ex: PB0-PB7), one tip that might be useful for you is the Add Pin Array function in the Symbol Editor which will allow you to mass-create all those pins at once.
Here are some tips on the Symbol Editor:
1. First enlarge the boundary box (Edit > Resize Boundary Box)
2. Now you can begin adding/repositioning/renaming pins
* Pins are always tied to the boundary box outline
Hopefully this gets you started!