OK I will assume that you have an Employee Report. You probably wrote the report using a Wizard. If that is so then you probably have a SQL string as the RecordSource for your Report. If that is so, then you can view it's contents by looking at the Property sheet. In order to reference the values within a table field they must be present in the records that make up your recordsource. So you should find something that looks like....
SELECT [tblEmployee].[First Name], [tblEmployee].[Last Name]...
If these are there then you can refer to the values that they store with the syntax I listed earlier
me.Full_Name = [First Name] & " " & [Last Name]
What Rich said is correct. If Access can not recognize the names from the list of names it finds in the RecordSource, then it will prompt for a Value.
If the First name and last name field are not included in the RecordSource of the Report, you could just add them right (be careful of the syntax), or you could write a whole new SQL Query with the Query Builder and cut the SQL string from there and paste it into the RecordSource property of your Report.
Any trouble, drop me a line
Chris