General Sewing supplies:
machine in good working order, foot pedal & plug (yes, people forget these!) - if you need to and have time, take it in for a tune-up before the retreat!
extra machine needles
any machine accessories you need (walking foot, FMQ foot, extension table, attached light, etc.)
depending on the setting, a small light for your area (not all rooms or all spots in the room have great lighting)
good scissors - large for cutting fabric, small for trimming threads
rotary cutter & extra blades
most useful rulers - usually a 6 1/2" x 24 & a square up ruler (6 1/2" or 9 1/2" or maybe 12", depending on what size blocks you plan to use) .. remember any specialty rulers you need for projects
cutting mat - 18" x 24" is good size to have at your table, usually there are a few large mats provided or shared by others at cutting stations
iron & ironing board (maybe - some places provide them or only permit a few limited ironing stations)
seam ripper, pins, pin cushion, hand sewing needles, safety pins
marking pens, permanent fabric pen, pencil, scratch paper (graph paper works great for last minute designs & quick sketches)
neutral thread for piecing - maybe a couple of light neutrals and a couple of darker neutrals
prewound bobbins w/ your neutral threads & empty bobbins for if you use a different thread
surge protector
If you are not participating in retreat projects, plan some projects to take. Pack the supplies for each project in its own ziplock bag. If possible, pre-cut any fabric. Make sure you include the fabric, the pattern, any templates that you need, thread (if you can't use the neutral), pre-wound bobbin to match, and any other notions that you need for the project. This way you can just pull out a ziplock bag and start working.
If you are participating in a retreat project, package the supplies for each project in the same manner.
most retreats have a snack table - so bring something yummy to share.
plastic cup to have a drink at the table (but be very careful w/ food & liquids around sewing areas!)
painters tape
duct/packing tape - to tape down any wires that might be in the way.