Depending on how you installed it, you'll have to configure it and open up the firewall.
http://msdn.microsoft.com/en-us/library/cc646023.aspx
If you hit all the defaults in SQL Server Setup, then you just need to open up a firewall port. Inside the VM, you can type this into the CMD prompt:
netsh firewall set portopening TCP 1433 "SQLPort"
TCP 1433 is the default port. The default SQL server instance I think is called MSSQL.
If you also want to allow the Browser Service (So you can browse the available SQL instances rather than trying to access them by CLI), then you'll need to open port UDP 1434.
`netsh firewall set portopening UDP 1434 "SQL Browser"
Or you can just go EZ mode and open everything SQL Server uses:
@echo ========= SQL Server Ports ===================
netsh firewall set portopening TCP 1433 "SQLServer"
netsh firewall set portopening TCP 1434 "SQL Admin Connection"
netsh firewall set portopening TCP 4022 "SQL Service Broker"
netsh firewall set portopening TCP 135 "SQL Debugger/RPC"
netsh firewall set portopening TCP 2383 "Analysis Services"
netsh firewall set portopening TCP 2382 "SQL Browser"
netsh firewall set portopening TCP 80 "HTTP"
netsh firewall set portopening TCP 443 "SSL"
netsh firewall set portopening UDP 1434 "SQL Browser"
netsh firewall set multicastbroadcastresponse ENABLE