For the first question, see ?graph.data.frame
and ?read.csv
.
I am not quite sure what you are asking in the second question, what is the error you are getting. Your code works fine for me, with the modification required for igraph 0.7.x:
V(metro)[sp$vpath[[1]]]
# Vertex sequence:
# [1] "Canal" "CuatroCaminos" "Alvarado" "Estrecho"
# [5] "Tetuan" "Valdeacederas" "PlazadeCastilla" "Chamartin"