Go to the Docs Folder
Open CustomEquipment.ods (using either Open Office Calc or Microsoft Excel)
Select the correct Worksheet for the type of item you want to add (equipment for equipment). Ignore the fact that it says "Mech Only" on the worksheet.
Select an empty row, and enter the data for the item you wish to add to SSW (At the top of the WeaponsMechOnly sheet is a legend for each column).
Save the file.
Depending on which program you use, the process differs at this point.
(The steps from now on are the same if you're using open office or MS Office)
Close Office (either one)
You should now have a file called customweapons.csv (or customequipment.csv, or customphysicals.csv or customammunition.csv) in the Docs folder of SSW
Open the file using Notepad, or similar text editor. Delete everything up to the name of the first piece of equipment, so your first line should be a piece of equipment (or weapon). Exit notepad, saving the changes (retaining the csv file type)
Go to the main directory of SSW, double click "binconvert.jar" The source file is the .csv you just created. Click the ellipse button and navigate to the file and select it.
Using the radial buttons, select the correct file type that you're using. Click Convert.
If you've done everything right, you should get a message about writing to a new file (Such as weapons.dat). Navigate to that file, and then move the new file to the Equipment folder, overwriting the old file.
If you have different types of equipment you wish to add (such as a weapon and its ammunition), you'll need to create .csv files for each worksheet. So for the weapon you follow the instructions from Step 6 with "WeaponsMechOnly" worksheet selected, and then you'd do the same steps with the AmmunitionMechOnly Worksheet selected.
Start up SSW, it should read the changes in the equipment file, and your new item should be there. Unfortunately at this time, you'll have to redo these steps with each new release.