Hello Mukesh Choudhary,
I might have misunderstood your question but Bootstrap multiselect is a widget which helps in selecting a Many-Many association. This is great when we want to do away with the Mendix Default Reference Set Selector and Input Reference Set Selector and want to select the association via a drop drop approach.
I am not quite sure when you mean “I have few values in an attribute, now I want to put these values in Bootstrap Multi Select Widget”
Can you please post a few screenshot and what you want the end output to be so that we can figure out if any other widget can help your purpose.
I suspect that your problem is that you don't configured the Multi Select Source and/or the Display Template and Template attributes.
If you have all this defined correctly you will see the data on the Multi Select.
I had the same problem in my project and was because the Display Template and Template attributes where missing.