In general, you cannot FORCE them to enforce immigration law. I presume you would like to compel the feds to enforce sanctions and prohibitions regarding the presence and employment of said illegal immigration.
In general, enforcement is a political question and not a legal one. There are few laws that REQUIRE law enforcement (local or fed) to act. To my knowledge, there is no law requiring the feds to detain or deport illegal immigrants.
With the current debate in Congress on what amounts to amnesty and the legalization of some 12 million illegals, morale in the USBP is very low. Many are effectively standing down effectively asking why they should risk their lives to enforce a law that may well be effectively moot in a few months. And make no mistake, these agents get shot at and assaulted every day ... unless you live in a border region, these stories do not make the news. When I lived and worked as a cop in San Diego, there were many such incidents, and may injured and wounded USBP officers.
Sadly, you won't generally be able to force them to enforce the laws as they are written.
- Carl