Dell as well as several other "off the shelf pc" retailers do not publish sufficient information about pc components installed in their computers. In addition there might be several different configurations.
The first problem with the Optiplex 990 is the pc case. There are 4 difference size cases from ultra small form factor (USFF) up to a minitower (MT) case. Which pc case do you have?
The second problem is the availability of a PCIe 2.0 x4 slot on the motherboard. The Optiplex 990 comes with an extra PCIe 2.0 x4 slot on the motherboard but it may be populated with one of several optional add-in cards. Do you have a free PCIe 2.0 x4 or better slot available?
In the meantime, here is a link to a short review of a new adapter card:http://www.maximumpc.com/article/%5Bprimary-term%5D/apricorn_velocity_solo_x2_pcie_ssd_adapter_review