CaddyShack,
Please check the system requirements at
Apple - Apple TV - Tech Specs. Note that one of them is:
"AirPort Extreme, Wi-Fi 802.11b, 802.11g, or 802.11n wireless network (wireless video streaming requires 802.11g or 802.11n), or 10/100BASE-T Ethernet network"
A USB modem connected directly to a Mac does not meet this requirement -- the USB modem provides an Internet access point but not a network.
Your Macbook can provide an 802.11 wireless network. To set this up, go System Preferences > Sharing > Internet tab & enable "Internet Sharing." If you need help with this, just click the "?" in the purple circle right in the preference window.
Note that you must have an Airport (802.11 networking) port enabled in System Preferences > Networking > Show: Network Port Configurations on your MacBook for this to work, & that the network will exist only when the MacBook is active (not powered down or asleep).
For this reason, people use Internet Sharing mostly with desktop Macs, or buy a router to use with a standard (Ethernet port equipped) cable or DSL modem, which is a far better & more flexible network solution. This option may not be open to you if you are using something like Sprint's EVO Internet access rather than a cable or DSL based service.