Code X means that the brokerage house doesn't know the holding period for your asset and you have to choose it from the information in your records. This happens most commonly on rolled-over accounts because the brokerage doesn't have the data from your previous broker.
Here's how to correct your holding period. After entering the sale info from your 1099-B - including your correct purchase date - click the blue button at the bottom, I'll enter additional information on my own.
On the page Let's get the rest of the info from your [Name] 1099-B, scroll down to the topic Corrected holding period and choose L - Long term (more than one year), or S - Short term (less than one year) then Continue.
Be certain you keep the supporting documentation with your return to prove your purchase dates, as using a different holding period can trigger a letter from the IRS.
If you know the date you purchased a stock, you can look up the price online at a site such as yahoo finance. If you don't know (or can't prove) the cost basis, the IRS considers it to be zero.
