A little hard to follow your question, but usermeta comes from the user profile of the wordpress user.  Are you saying that your trying to bring data from 1 form into another?  Or from the users profile?

okay sorry for my bad explanation....
i`ll try to load checkboxes with userdata usermeta values
The data are stored as single value like
Sport1
or multiple values 
Sport1, Sport2, Sport3
and with this values i want to prechecked one or multiple Checkboxes with the [usermeta key="UserMetaKey"]
but it`s not working

any ideas ?

Having looked into the problem, I can confirm that if the responses are stored in user_meta and the choices/values in the checkboxes/database contain commas, the pre-filling of fields will not work.
For example, these choices/values will work:
  --
Yes I would like to take part in the event
No I don't want to take part in the event
--
But these choices will not work:
--
Yes, I would like to take part in the event
No, I don't want to take part in the event
--
I really hope this is of use to someone, I've wasted a lot of time finding out where the bug came from.

Line breaks are not interpreted in replies to this forum, I had to insert br tag manually.