The Peer Edge Locate command is not available unless you also set the Sketch Relationships to Peer Edges option on the Inter-Part tab on the Options dialog box.

When peer edge locate is active, you can place dimensions and relationships between sketch geometry and the edges of other parts in the assembly, but you cannot locate peer edges when creating new sketch geometry. For example, to place a line to the endpoint of a peer edge, you must first place the line near the endpoint of the peer edge, then use the Connect command to add a connect relationship between the endpoint of the line and the endpoint of the peer edge.

You can use the Select All Peer Relationships command on the shortcut menu to select all the dimensions and geometric relationships in the current sketch that reference other parts in the assembly.