To populate a numeric field (i.e. Float, Decimal or Integer type) with random numbers (e.g. values between 0 and 100), use the Table> Update Column tool (ensure that the table is editable i.e. in a native MapInfo format)).
Set the target table in the Table to Update area, and the target field in the Column to Update area.
In the Value area, enter rnd(1) - this will generate a decimal fraction (i.e. values between 0 and 1).
This can be modified to create more specific ranges:
rnd(1) * 100 - creates values between 0 and 100
rnd(1) * 10 +10 - creates values between 10 and 20