Try the the ';' character to separate the masks for positive and negative values:
textEdit1.Properties.Mask.EditMask = "+#0.00;-#0.00";
textEdit1.Properties.Mask.MaskType = DevExpress.XtraEditors.Mask.MaskType.Numeric;
textEdit1.Properties.Mask.UseMaskAsDisplayFormat = true;
Related link: Mask Type: Numeric