Is a H-2A employee eligible to enroll in their Employers health Plan

We don't know what the plan is or what state. Your employer has to answer this.
California probably has the most options, and has a plan called Covered California for farmworkers.
Soon or maybe even right about now, you might have more options under Medi-Cal.
A lot is new under the Affordable Care Act (Obamacare).
http://www.cpca.org/cpca2013/assets/...-FactSheet.pdf

That is considered a Temporary worker, not a Permanent or full time employee... and I can find nothing that specifically says they are entitled to that, unless its offered by the employer. So you have to ask your employer. Most plans however are for year long terms, and few if any H2A jobs are for anywhere that long