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.
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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.