This simple script might work if the order of the columns is Subject, Date, Body and Category. Note: You have to select the sheet in the frontmost window.
tell application "DEVONthink Pro"
set theSheet to content record of think window 1
if type of theSheet is not sheet then error
set theForms to children of theSheet
repeat with theForm in theForms
set theFields to cells of theForm
set theSubject to item 1 of theFields
set theBody to item 3 of theFields
set theCategory to item 4 of theFields
set theGroup to create location "/" & theCategory
create record with {name:theSubject as string, type:txt, text:theBody as string} in theGroup
end repeat
end tell