Here is what I suggest you try to do to add it manually.

Open the file using WinZip or WinRAR.
Find and open the instructions text file (often named ReadMe.txt) with the instructions.
Find the texture.XXX folder and drag and drop it into the FSX\SimObjects\Airplanes\CS_707-300 folder.
Open the 707-300's aircraft.cfg (using Notepad). BACK IT UP FIRST! Just in case you mess it up.
Copy the lines from the instruction text file. They look like this:

[fltsim.xx]
title=CS_B707-300 Qantas V-JET
sim=csx701
model=adv
panel=
sound=
texture=QVJ
kb_checklists=
kb_reference=
atc_id=VHEAD
atc_airline=Qantas
atc_flight_number=703
atc_heavy=1
atc_parking_types=GATE,RAMP
ui_manufacturer=Boeing
ui_type=707-300B ADV
ui_variation=QANTAS V-JET 707-300
ui_typerole=Commercial Airliner
ui_createdby=Captain Sim
description=707-331B QANTAS Repaint by nath2889, 1977\n707 Captain (707-300) 1.3\n(c) 2010\nCaptain Sim\nwww.captainsim.com\Repaint by nath2889

Paste it in the aircraft.cfg, below the last [fltsim.XX] entry in there. In my case it is [fltsim.14]
Rename the [fltsim.xx] (in my example above) to [fltsim.15]. It would then look like this:

[fltsim.15]
title=CS_B707-300 Qantas V-JET
sim=csx701
model=adv
panel=
sound=
texture=QVJ
kb_checklists=
kb_reference=
atc_id=VHEAD
atc_airline=Qantas
atc_flight_number=703
atc_heavy=1
atc_parking_types=GATE,RAMP
ui_manufacturer=Boeing
ui_type=707-300B ADV
ui_variation=QANTAS V-JET 707-300
ui_typerole=Commercial Airliner
ui_createdby=Captain Sim
description=707-331B QANTAS Repaint by nath2889, 1977\n707 Captain (707-300) 1.3\n(c) 2010\nCaptain Sim\nwww.captainsim.com\Repaint by nath2889

Save it. You should now see the aircraft in FSX.

If that's too difficult for you to do (or you are not confident doing it), just tell me the livery you are adding (and where I can download it) and I'll see if I can set it up to be installed using ACE.