Trying to Run a Report...What am I doing wrong?

It sounds like you may be missing the DiskDrives table in the from clause of the SQL.  You can add it like this:

from Computers join DiskDrives using (ComputerId)

I attached a file of what I am doing. I added the line but I am getting a syntax error when trying to run it.

I was able to get it to work, however, when it displays it does not display in GB.

The size column is in bytes, so you need to divide it to make it GB:

select
 Computers.Name as "Computer Name",
 Computers.Description,
 DiskDrives.Size / 1024 / 1024 / 1024
from Computers join DiskDrives using (ComputerId)
where <ComputerFilter>

Thank you very much for the quick response! One last question. If I also wanted to get a list of applications how would i create the column for applications?

You would use the Applications table like the DiskDrives table.  

select
 Computers.Name as "Computer Name",
 Computers.Description,
 Applications.Name
from Computers join Applications using (ComputerId)
where <ComputerFilter>

I figured out how to do it for applications! Thanks for all your help!

No worries.