Having reported to General Tullius, you will be sent to Pale Imperial Camp. Once again, at the camp, you will want to report to Legate Rikke. She will tell you that they need to get their hands on some Stormcloak Orders.
Find the Stormcloak Courier
Inside either Inn, you will want to talk to the Innkeeper: Elda Early-Dawn in Candleheath Inn and Hadring in Nightgate Inn. If you have a Speech 50, you will be able to succeed in the Persuasion. You can always succeed by Intimidating and can usually succeed with bribery. The Nightgate Inn is recommended for its remote location. The Candlehealth Inn can be used to track down the courier, but you will need to follow him out into the wilderness or attempt to pickpocket him inside the inn.
This will prompt them to tell you that the Courier should be back in a while. Wait an hour and the courier will likely appear.
If you talk with the innkeeper, this will make it easier for you to track the courier if you head outside as well. They will be marked at this point.
Retrieve the Stormcloak Courier's package
You can either watch him and wait for him to head either into the wilderness. Inside the Inn, wait for him to either leave or go to bed.
In the wilderness you can ambush him or talk with him and then engage him.
Taking down the Courier, loot him and retrieve the Stormcloak Documents.