Hi!
Don't let the "Flexy" name mislead you
LiteCommerce 2 (2!) used an early version of the PEAR's package named Flexy. But the code changed a lot since then, so now it is a different beast.
Use this website instead:
http://kb.x-cart.com/dosearchsite.action?queryString=flexy
As for the link on the help.x-cart.com website that Mike mentioned: it is for the Flexy version used in LiteCommerce 2. Go there only if you can't find information in the official XC5 dev guide at kb.x-cart.com - there are many improvements in the Flexy version used in X-Cart 5 which aren't even mentioned in the LiteCommerce 2 docs.
If you have a specific question feel free to ask in these forums. Tony from the XC5 team does a good job consulting third-party developers there.