Tips:
-Run game as Administrator, check how to do this clicking here.
-Case the mod menu don't appear after selecting it on the NIBMods menu, maybe its a issue related to the characters, suits or armors .ini files in Scripts\Characters (suits or armors) folder, delete all character .ini files from that folder and in game press Insert to reload scripts.
-You need internet access to login validation (if needed by mod)
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Know issues that make the NIBmods menu don't work when you press Ctrl + N.
Obs.: I'm considering that you have the ScripthookV, ASI Loader and NIBSHDotNet installed. Case you didn't installed them yet, check this post.
Those issues are commonly related to problems with the NIBSHDotNet plugin, basically what can happen is:
Issue #1: ASI Loader fails to load NIBSHDotNet
If even with correct version of NIBSHDotNet your mods menu still don't load it may be result of NIBSHDotNet not even being loaded, to confirm this case open the asiloader LOG file and see the line that refers to the NIBSHDotNet, if you find something like this:
Loading "...\Grand Theft Auto V\NIBSHDotNet_loader.asi"
"NIBSHDotNet_loader.asi" failed to load
SOLUTION: You need to check if your windows have the required windows packages installed and working, the packages are:
-Microsoft .NET Framework 4.5 (direct link)
-Microsoft .NET Framework 4.5.2 (direct link)
-Visual C++ Redistributable Package for Visual Studio 2013 (x64) (direct link)
-Visual C++ Redistributable for Visual Studio 2015 (x64) (direct link)
Issue #2: Scripts not in scripts folder
SOLUTION: Make sure that the scripts (.dll or .ndll files of the mod) are in the gta Scripts folder, otherwise the NIBSHDotNet can't find and load them. My mods goes with a ZIP file with the structure that you can simple copy and paste in game root folder for easy installation or come in .OIV format for easier setup using OpenIV package installer.