Rule #1: There is no magic. When someone sources a file containing a proc and this proc does not exist later, then either someone deleted it or loading the file went wrong. This has not much to do with intuition or experience.
Concerning the error message. My distance diagnosis guess is that you have installed tcllib at a place, which is not included in the load-path of tcl (within aolserver) and that you have a version of openacs installed which is older than 1.5 years.
Why do i think so?
There was a file acs-tcl/tcl/mime-procs.tcl in OpenACS, which was removed 1.5 years ago. This file started with the lines:
if {[catch {package require mime}]} {
package require Tcl 8.3
package provide mime 1.3.3
...
How do i find out, what the package-path is?
Go to developer shell (ds/shell) and type there
set ::auto_path
Hope this helps.
-gustaf neumann