Hello Ernestine,
I took a longer look at this import as I was unsure myself about the fields. To import an activity, you need three fields: AssignedTo, ActivityType and DueDateStart. You can find which fields are required by going to Customize -> Tables/Fields -> Activity. You can scroll through the fields seeing which ones are checked as Required?. I would recommend three others: DueDateEnd, Contacts and Entity. The Contacts field will be the contact assigned to the activity and the Entity will be the name of the contact's customer name.
You should have the Leads imported already. When you set the Contacts and Entity field for the activity record, it will set the customer's information in the activity. It gets the information from the Customers table (which holds Leads) and fills in the activity on the import.
Below is my basic import for an activity. Notice the Contacts field is the RecordID for the contact to be on the activity. The Entity is the contact's company name since the Entity field is the FullName from the Entity table.
Here you can see the Entity FullName for the contact Mary Swanson is Mary Swanson from the Customer Name field. The Contact 'Mary Swanson' has a RecordID of 171 in the Contacts table.
In the end, the process is..
1) Import the Lead company/customer. Check out this blog post here.
2) Import any contacts for the lead company imported.
3) Import your activities linking to the leads.
Greg