EDH Help (U/R)

Seems a little creature light. Also, how do you win? With very few creatures in the deck, Nin only gets to draw your opponents cards. I guess you can blast her yourself but that just doesn't seem like it would get you anywhere. I'd consider things like windfall and molten psyche to punish them for having so many cards or atleast keep you from being buried by CA. Aeon Chronicler would be really good in here, as would Duplicant, Draining Whelk, Sower of Temptation, and Dominus of Fealty, so you can take their dudes, punch your opponents with them and blast them to death drawing a million cards. Consecrated Sphinx is also an auto include. Solemn Simulacrum is something to consider aswell with all the upside in killing him yourself.

I'll have to agree on a mana rock package (Mana Vault, Sol Ring, Izzet Signet atleast) because you want those big X's. Tezzerret, the Seeker and Trinket Mage work well with these and open up a nifty Trinket Mage package so you can hate out problem strategies (Relic of Progenitus for GY decks, for example or Expedition Map for Strip Mine or Wasteland to take care of problem lands or Minamo, School at Water's edge to untap Nin and machine gun down some dudes.)

Don't run Mana Leak either as its really bad when people have a lot of ramp which is common in EDH. Memory Lapse is also pretty bad when you can run things like Counterspell, Dissipate, Cryptic Command, Hinder, and Spell Crumple.

Nin is an interesting commander, for sure, but I feel like she requires a plan to actually win the game. I was looking at your list wondering what drawing a million cards does for you or what your deck even does. The synergy just isn't there. You could run ANY other UR commander under this configuration (Looking at you, Keranos). TNN beats is about all I see here.

On your sideboard:
Stifle is strong in commander. Ponder, Brainstorm, Preordain should be your draw package over things like Serum Visions. Top and Needle would be fine. Once again, I'd avoid any counterspell that says gives your opponent an out with mana. It's just not as good as it would be other formats. Keranos seems good, and Clique is always good but that's all that I would consider auto includes from that list. Oh and Forbid, that things seems pretty sweet in your list. Misdirection might be something else to consider with you drawing so many cards theoretically.

Sorry for the giant wall of text, but hopefully this helps.