Cimatron offers several ways to design inserts for molds.  The choice you make depends on where you are at in the design and your thought process about the the insert to be made.

a). Inserts can be considered from the start, just like core and cavity.  You can use Quick split to separate areas of the part
that will be inserts, and then use the cut active function to create new parts.

Often however insert design is not a high priority at the beginning of the design.  Only later do we consider where inserts go.  
This is where you will use the insert tools the mold package offers.  There are basically 2 ideas here:
b). Add Insert is an all inclusive insert making tool, from sketches or surfaces in the cavity.  There are really quite a lot of options
    to it - see the help for what you can do.  You can even build a cutting object for the new insert while you are making it.
c). There is also Add Insert from catalog.  This is more useful for a standard part like a core pin or something already created.  You
    can add it to the design and have it use it's cutting object for the insert pocket.  With Add insert from catalog, you want then use
    Cut Insert as the next step.  This puts the part shape on the insert, direct from the parting area.

d). There is also a very generic function that all assembly users have, Extract by contour.  This allows you to divide an existing part into 2 parts
based on a sketch you make.  Essentially that is an insert making function, but it has no real smarts and is a harder update path to use
when you experience an ECO or design changes.  Avoid this function for insert design.