How do I gain access to the R&D Lab?

As per the Game Dev Tycoon wikia page you need to be in the third office (I'm assuming you're not counting the starting 'garage' as an office, in which case you'd need to be in the fourth "office") and meet the following requirements;

Must have access to the third office and have at least one designer specialist. Each additional designer specialist will reduce the cost of development (by 100K each at maximum rate).

The "third office" is the office that looks like this;

In order to get a designer specialist, you must train at least one of your employees to have 700 in design, followed by the 'Designer Specialist' course (available in the training menu).