Make a clock, then put a command block under where the player will be. The command will be:
/testfor @p[r=0] replace the 0 for your radius.
Place a comparator running out of the block then into another command block.
Command will be:
/say Hello
Or
/me Hello

Thanks,
Bye!

/say doesn't accept a player parameter, it will just say a message to everyone. If you want to tell a message to certain players, either use /tell @p[r=6] <message> or /tellraw @p[r=6] {FancyMessageStuff}. The /tellraw is harder to use, but you can put different effects and colors on strings of text, and it just overall looks nicer.

If you do go with /tellraw, this website is great and puts the command together for you. http://ezekielelin.com/minecraft/tellraw/

If your in 1.8 (or the newest snapshot) you can make a redstone clock connected to a bunch of command blocks to test for a player and use a redstone comparator to output a signal when a player enters the area, the best part is that you cant set if off again so its easy to have the event happen once but if you back track it wont have the same moment from the adventure map happen again.
If you ever need any help with more redstone problems or need me to make a video to show how to do it. you can contact me via.
Twitter: https://twitter.com/Somewhat8
YouTube: http://www.youtube.com/user/SomewhatInfinite
Or my planet minecraft (I dont get on this one as much)

Hope I helped

Pressure plates using /testfor might work, but I'm not sure if that counts as "wireless".