The poem is addressed to John Milton, who is dead, the figure of speech known as apostrophe is used.  

"Apostrophe, figure of speech in which an absent person, a personified inanimate being, or an abstraction is addressed as though present." 

There are similies and metaphors in this poem.  A similie is when you compare two unlike objects using the words like or as, a metaphor is when you compare two unlike objects and you say that the object is, for example, in line two, England is compared to a fen "she is a fen".

An example of a similie is in the line, "thy soul was like a star." 

Milton is compared to be "as pure as the naked heavens." This is a similie because of the use of the word as. His voice was like the sea, similie.

"Milton! thou shouldst be living at this hour;
England hath need of thee: she is a fen
Of stagnant waters: altar, sword, and pen,
Fireside, the heroic wealth of hall and bower,
Have forfeited their ancient English dower
Of inward happiness. We are selfish men;
Oh! raise us up, return to us again;
And give us manners, virtue, freedom, power.
Thy soul was like a Star, and dwelt apart;
Thou hadst a voice whose sound was like the sea:
Pure as the naked heavens, majestic, free,
So didst thou travel on life's common way,
In cheerful godliness; and yet thy heart
The lowliest duties on herself did lay." (Wordsworth)