Okay. That tells me that it needs to be added using ACE. This is a partial copy of the instructions found in the ‘C-130 X-PERIENCE’ FLIGHT MANUAL Part I – User’s Manual (cs131_manual1.pdf), pages 10 &11:

2. CREATE A LIVERY OF YOUR CHOICE.
IMPORTANT:
The new livery set should be ACE (Aircraft Configuration Editor) compatible. Make sure the new livery set meets the following requirements:
- Texture files are not renamed; - Texture files are in DXT format; - Package includes 256x128 pixels airlinenameYYYY.JPG image (screenshot) of the new livery. - Package includes livery.cfg (do not rename it!) file in the following format:
-----------------
[livery]
model = 0 (model ID, see below)
0 = C-130E 'Hercules'
1 = C.Mk1 'Hercules'
2 = HC-130 'Hercules'
3 = C-130J 'Hercules'
4 = C-130J 'Hercules'
5 = C-130T 'Hercules'
6 = CL-130 'Hercules'
7 = KC-130 'Hercules'
8 = C-130A 'Hercules'
9 = C-130H 'Hercules' French
10 = C-130H 'Hercules'
11 = C-130H-30 'Hercules'
12 = L-100-30 'Hercules'
13 = LC-130 'Hercules'
14 = NC-130 'Hercules'
15 = AC-130 'Spectre'
16 = C.Mk3 'Hercules'
17 = C-130J-30 'Hercules'

texture= short (5-7 characters) but unique code of your repaint.
For example: for Greece Air Force livery circa 1980 you’d use ‘gaf_80 code’. The code will be used in the texture folder name in the MSFS: texture.gaf_80

atc_id = XXXXXX any numbers & characters

atc_airline = XXXXXX any numbers & characters

atc_flight_number = XXXXXX any numbers & characters

ui_variation = Unique variation name. Should include full name and year when the livery was current AND repaint artists' initials. For example: Greece Air Force, 1980 FH

atc_id = XXXXXX any numbers & characters

description = Description of the livery. Any info you'd like to provide on the repaint. Artist name and e-mail address – recommended for feedback.
\n(c) 2007 Captain Sim www.captainsim.com line must be included in the description.

EXAMPLE OF THE LIVERY.CFG FILE:
------------------
[livery]
model=0
texture=ba_00
atc_id=C001
atc_airline=USAF
atc_flight_number=0001
ui_variation= Blue Angels, 2000 AP
description="C-130E Hercules (c) 2007 Captain Sim www.captainsim.com"

I suggest you read it to see if it helps. If you get stuck, email me with the name of the livery and where you got it (URL) and I will set it up to work for the FSX model. I'll email it back to you completed. It's the best I can do and it is easier than going through the complexity of exactly HOW to do it. ;)

Mark