Depends on where you put pkg-config and what configuration options the program you want to install takes.
Try"
./configure --help | more
which will tell you what configuration options there are.  It might let you give library paths or include paths.
If pkg-config is a library, you need to update your libraries.  Edit ld.conf to make sure the library path you are using (ie /usr/local/lib) is in there.  If not, add it.  Then run ldconfig to update the library paths.
Also, you could export a variable telling it where pkg-config is.  Use the setenv command for tcsh (on a Mac) or export for bash.
********
whereis is a command that sorta works like find.  It doesn't find normal files, though.  It only finds executable files and paths.  It's a completely harmless command, and did not fuck up your system.