In order for you to use the market, there must be a way to communicate with other participants. This can be done by using either bitmessage or email. Bitmessage is used to post offers to the market, and has to be enabled if you want to trade. The downside with bitmessage is that it can be slow sometimes. Email is faster, and to speed up communication with others you can enable that as well.
To enable bitmessage you only have to tick off the box. Your address will be generated by the software.
For more information about bitmessage go to geek section