When you do
app target->build phase->Link Binary With Libraries
you see those libraries/ frameworks which are available in iOS SDK.
To link to the library created by you, click on +
sign at the bottom left and then Add Other..
Browse to the .a file you have created, Click Open.
PS: To save your .a extension, after building your Cocoa Touch Static Library
project ,
under Products, you will see, .a file. Right Click-> Show in Finder -> Copy to some location