Having problems with using the mods button on the pip boy

Here's the revolver in question:

The Mod button is highlighted in the top right. When you press X the below menu should appear (forgive me for having only one mod, couldn't find another one).

When you click on a mod, it will be attached to a weapon (it will be noted with a "+" next to its name, and the attached mod will be listed in weapon's properties panel:

Make sure you have appropriate mod for a weapon - it's usually obvious from a mod's name which weapon it's for. Also, since you tagged this question as "pc", I assume you're pressing X on keyboard and not on a gamepad.

I don't remember having to use the keyboard (maybe that's the problem; you can't), click on it with the mouse instead. Same goes for if you have a controller: try the mouse. Also, some guns will only take certain mods, and it's either a bug or something, that keeps mod lit up even though the mod won't go on it, as I've had that happen to me, too. I do however have a bunch of mod files installed, including Fallout3, in its entirety.

There's only two mods for the 357 (falloutwikia):

• .357 magnum revolver long barrel - increases the weapon's damage by 3.
• .357 revolver heavy duty cylinder - increases the weapon's durability by 50%.

For GRA Merged: (nexusmods.com)

All weapons have had their tags (GRA, Mad Bomber, etc) removed, and vanilla versions of weapons use the GRA weapon mods instead.

It's Page Up. After a firm hand on the keyboard and swiping left and right, finally figured it out.