First install dependencies from FreeBSD package tree:
For x64 that would look like:
pkg add http://pkg.freebsd.org/freebsd:10:x86:64/release_3/All/net-snmp-5.7.3_11.txz
pkg add http://pkg.freebsd.org/freebsd:10:x86:64/release_3/All/neon-0.30.1.txz
pkg add http://pkg.freebsd.org/freebsd:10:x86:64/release_3/All/nut-2.7.3_3.txz

Then add the package
pkg add <url>/pfSense-pkg-nut-0.1-x64.txz