I m not able 2 enter the Mass allocation Formula even after changing the profile Option and getting some error message related to flexfield dat were talked about in d video.though i hav sucessfully created the OUs as in payables videos ,plzz help

Please always post the error message in order for me and others to understand what is going on. Otherwise how would I know which error did you get? Also please always inform the audience about "when" did you get this error exactly. E.g. after pressing a button or after trying to save the work etc.

Can you verify two things:
1) Your chart of account structure segments are using the sequence 1,2,3 (and not 10,20,30.)
2) You have created the OU and attached it to your GL responsibility using MO: Operating Unit profile option.
Thanks