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.

You can create the description.txt yourself. Just follow these guidelines:

3. CREATE DESCRIPTION.TXT
Do not modify text shown in bold font!
Do not type text shown in grey.

[texture]

model = for the 707-300 and 707-300C can be one of the following below:
· adv
· b
· c
· 300

E-3 must be model=e3

VC-137 must be model=137

ui_variation = Unique variation name. Should include full name and year when the livery was current AND
repaint artists' initials. For example: Lufthansa 1980 JS

description = 707 Captain (Your description of the livery). Free livery, repaint by Your Name. E-mail
address is recommended. \n (c) 2010 Captain Sim www.captainsim.com

atc_id = XXXXXX (any numbers & characters)

atc_airline = XXXXXX (any numbers & characters)

atc_flight_number = XXXXXX (any numbers & characters)



EXAMPLE OF THE DESCRIPTION.TXT FILE:

[texture]
model = adv
ui_variation=Lufthansa 1980
description=707 Captain (Lufthansa, 1980). Free livery, repaint by John Smith. \n (c) 2010 Captain Sim
www.captainsim.com.
atc_id=1234
atc_airline=1234
atc_flight_number=1234

The above instruction are quoted from the ‘707 Captain’ FLIGHT MANUAL Part I – User’s Manual (cs707_manual1.pdf) Page 36