I am level 31 and I have the Elder Scroll. I am not sure if my level is high enough to go to Whiterun and trap one of Alduin's minions. What level should I be before I attempt it?
Most of the story, including Alduin, scales very well with the player. You should not have a lot of trouble with any of the story encounters at any level, unless Bethesda meant for that particular part of the story to be hard (like the part leading to the final encounter).
One thing to keep in mind though, is that while enemy stats scale with the player's level in a way, things still can be made easier with equipment and spells. In other words, a character with top-tier equipment and lots of spells will have an easier job killing Alduin at level 30 than a character with cloth armor and no spells at level 60.
That particular part of the game where you are now shouldn't prove any trouble, because:
When you go to Whiterun, all you gotta do is pull a lever to trap a dragon. After that, you can enjoy a little ride. :)
I was level 28 when I killed Alduin, But I had full Daedric armour, which had been buffed to more than 1000 armour points, and twin Daedric Swords dealing about 300 damage each. You don't need to have a certain level, just skill up and use proper equiment for it!
You can stack potion effects you know, so multiple Draught of the Blacksmith and enchanting bonuses would really help in getting almost over-powered gear easily.
Good Luck killing Alduin!
The best advice when playing Skyrim is dont put all your skill points in crafting and running. Spread it out between offensive / defensive and crafting.
If you are level 28 and all you have done is Blocked your way through the game and craft, your going to be crap at fighting any of the Monsters and dishing out offensive.
Spread your points. IF you cant beat them stop crafting / smithing for a while and level up your Swords/ Shields\ Archery, etc.