"is not a member of package" error when importing package in Scala with SBT

Question

(Relative beginner here, please be gentle...)

I've got a Scala program that I can build with sbt. I can (from within sbt) run compile and test-compile with no errors. I've defined a package by putting package com.mycompany.mypackagename at the top of several .scala files. When I do console to get a Scala REPL, this happens:

scala> import com.mycompany.mypackagename._     
<console>:5: error: value mypackagename is not a member of package com.mycompany
       import com.mycompany.mypackagename._

Any variation of this also fails. When I just do import com.mycompany I get no problems.

I thought that running the Scala console from within sbt would properly set the classpath based on the current projects? What (completely obvious) thing am I missing?