Import : Documentation and advices ?

Import : Documentation and advices ?

I am trying to import contacts, tasks, notes from Act!.

My csv files seem correctly formatted. However 3 times out of 4, I receive a message from Zoho without any explanation :
"Error occured in Data Migration while migrating the data from the null file ".

Does it exist on Zoho a documentation or some advices webpage which could help me to import my data ?

Thanks in advance.

Mayeul