TblCustDevices stores device custom fields, not computer custom fields. You need tblCompCustom for computers.
To import the information you already have, you will have to write a custom script. An example script can be seen below. It updates the purchase dates of computers LAN-001 and LAN-002 and can be executed in the Lansweeper configuration console under Lansweeper\Database Scripts.
Some of our customers have also used Microsoft Access to connect to their Lansweeper databases, to more easily import data.
UPDATE tblCompCustom SET PurchaseDate = '08/27/2012' FROM tblCompCustom INNER JOIN tblComputers ON tblCompCustom.Computername = tblComputers.Computername WHERE (tblComputers.Computer = 'LAN-001')
GO
UPDATE tblCompCustom SET PurchaseDate = '08/28/2012' FROM tblCompCustom INNER JOIN tblComputers ON tblCompCustom.Computername = tblComputers.Computername WHERE (tblComputers.Computer = 'LAN-002')
GO