Spaced,
The best thing to do is to click the small coloured box to the right of the Table Array box and then go to spreadsheet B and highlight the whole data set on spreadsheet B - this is telling the function to refer to this table in the lookup.
What you have to remember is that the field that you are looking up to (reg numbers) in spreadsheet B has to be in column A of spreadheet B.
I'll give you an example of what the function should look like based on the following assumptions.
Spreadsheet A has 2 Columns (A & B) - Registration Number (A) & Mileage (B). Cell A1 contains the header "Registration Number" and cell B1 contains the header "Mileage". The sheet has 50 rows.
Spreadsheet B has 2 Columns (A & B) - Registration Number (A) & Name (B). Cell A1 contains the header "Registration Number" and cell B1 contains the header "Name". The sheet has 50 rows.
So in cell C2 of spreadsheet A you need to insert function :
=VLOOKUP(A2,SheetB!$A$1:$B$50,2,FALSE)
then fill this down in column C of spreadsheet A
Does that help ??
BW