Skyline has two different ways of determining the LOD. You can choose these at:
Settings > Peptide Settings > Quantification > "Calculate LOD by:"
The "bilinear" way involves Skyline fitting a horizontal line as well as a sloped line to your data points, and then if you choose "bilinear turning point" as your method of LOD calculation, then Skyline will tell you that point where the horizontal and sloped lines intersect is the LOD.
The other two choices for LOD determination are "blank plus 2 * SD" and "blank plus 3 * SD" which require that you have more than one "blank" sample in your dataset.
Do you have any Replicates in your Skyline Document where you have set the Sample Type to "Blank"?