16550 UART to Avalon

First of all you need an Avalon bus interface, because opencore modules usually interface to Wishbone bus, not Avalon.

Anyway the Wishbone/Avalon glue logic is fairly straightforward. You can refer to this thread and the links you'll find in it:

http://www.alteraforum.com/forum/showthread.php?t=2238

The next step is building the sopc component.

First of all read the guide http://www.altera.com/literature/hb/qts/qts_qii54004.pdf

Then, I'd suggest you take a tcl file from another component as a template and write your own, changing what you need. IMHO this is more convenient than creating the new component from scratch in sopc builder, but you can do either.

Remember that the tcl file must be *_hw.tcl, otherwise sopc builder won't find it when you'll refresh the component list.