When you are doing "Edit > Insert > Peptides", the "Description" column is read only.
If you have a background proteome (.protdb) file, then Skyline will look for the protein name that you have typed in and, if it finds a matching protein in the .protdb file, will fill in the Description column.
You can create a background proteome file on the "Settings > Peptide Settings > Digestion" tab.
One thing that you could do is use the "Edit > Insert > Peptides" to add a list of peptides without proteins to your document.
Then, you can use the "Refine > Associate Proteins" menu item and give Skyline a FASTA file, and Skyline will figure out which peptides belong to which proteins and rearrange the peptides in the document.