Assuming MS >= 0, here are three ways, in increasing order of compactness:
con Mycon {i in PickUpNode}:
(if i = 'Cary, NC' then 1 else 0)
<= sum{<s,d> in RouteTable: s=i} MS[s,d]
<= 1;
con Mycon {i in PickUpNode}:
(if i = 'Cary, NC' then 1 else 0)
<= sum{<(i),d> in RouteTable} MS[i,d]
<= 1;
con Mycon {i in PickUpNode}:
(i = 'Cary, NC')
<= sum{<(i),d> in RouteTable} MS[i,d]
<= 1;
You might find these examples useful:
SAS/OR(R) 13.2 User's Guide: Mathematical Programming Examples