Can anyone help me equipe the scope I bought for my 9? I can not figure this out.

Look for the button which says Mod on the pip-boy weapons screen, and hit it.

o.k. I got it and not that you probobly need the warning but. . .
*spoiler alert*

The problom was I was trying to place the scope on the gun I stole off Benny's dead body which is technicly a 9mm but doesn't work with the scope

Welcome to the world of disappointment that is unique weapons.

Sic: only none unique weapons can be modded.
"Mama always said special is as special does."
Any gun with a name that seems "special" like "Maria" or "Lucky" is unique they have a better profile than ordinary guns.
Often have a unique texture or mesh, and sometimes have upgrades already in place.

For mods you want to find guns named along the lines of "9mm", ".357", "hunting shotgun".. etc, regular boooring, commonplace weapons.
Also uniques are really silly and everyone uses them ( seemingly ) except me ( as I'm learly liceasp ), modded guns are more fun.

Weapon Mods Expanded

Version: 1.0.6
Date: 02-04-2011
Author: Antistar (Joseph Lollback)

9mm Pistol / Maria
* 9mm Pistol Extended Mags - Increases ammunition capacity (+7).
* Small Handgun Scope - Adds short-range scope.
+ 9mm Silencer - Silences the weapon.

(just one example of a bazillion great modifications this mod adds for most non-melee weapons in the game, including uniques)

Best mod evah. If you are on PC. If not, sorry.

With a fitting engraving on the scope

A link for your convenience: http://www.newvegasnexus.com/downloads/file.php?id=39651

The scope on 9mm is mostly a hindrance, anyway. At ranges this short and magnification this low, all the scope really does is limit your field of vision.

Ratslayer is good, for the gun it's based on. It's already modded.
And This Machine is a nice Unique...altho I mostly like that one because of the fun sound it makes.

Plus it truely is unique, not just a custom variation like most of the uniques.
I love the garrand sound, not a huge gun fan but that sound is very nice on the ears..
Well my ears and when not getting hit by bullets fired from one I should add for clarity.

If you're playing on the PC and don't mind deviating from Vanilla gameplay, there are several mods out there that let you modify unique weapons. Here's one of them: http://www.newvegasnexus.com/downloads/file.php?id=35009

If you want to go even farther and add tons of new modifications to the game, I highly recommend Weapon Mods Expanded, which I believe someone above me linked you to.

So the bottom line is, you cannot equip mods to unique weapons, such as Maria, the unique 9mm pistol found on Benny's body.

Fixed it for you