Each port is extracted in to a working directory, which must be writable. The ports system defaults to having the DISTFILES unpack in to a directory called ${DISTNAME}. In other words, if you have set:
PORTNAME= foo PORTVERSION= 1.0
then the port's distribution files contain a top-level directory, foo-1.0, and the rest of the files are located under that directory.
There are a number of variables you can override if that is not the case.
The variable lists the name of the directory that is created when the application's distfiles are extracted. If our previous example extracted into a directory called foo (and not foo-1.0) you would write:
WRKSRC= ${WRKDIR}/foo
or possibly
WRKSRC= ${WRKDIR}/${PORTNAME}
If the port does not extract in to a subdirectory at all then you should set NO_WRKSUBDIR to indicate that.
NO_WRKSUBDIR= yes