Returning the string between the 5th and 6th Spaces in a String

In Tableau 9 you can use regular expressions in formulas, it makes the task simpler:

REGEXP_EXTRACT([Description], "Target Name: (.*?) ")

Alternatively in Tableau 9 you can use the new FINDNTH function:

MID(
     [Description],
     FINDNTH([Description]," ", 5) + 1, 
     FINDNTH([Description]," ", 6) - FINDNTH([Description]," ", 5) - 1
   )

Prior to Tableau 9 you'd have to use string manipulation methods similar to what you've tried, just need to be very careful with arithmetic and providing the right arguments (the third argument in MID is length, not index of the end character, so we need to subtract the index of the start character):

MID(
   [Description]
   , FIND([Description], "Target Name:") + 13
   , FIND([Description], " ", FIND([Description], "Target Name:") + 15)
     - (FIND([Description], "Target Name:") + 13)
)

Well, you need to find "Target name: " and then the " " after it, not so hard. I'll split in 3 fields just to be more clear (you can mix everything in a single field). BTW, you were in the right direction, but the last field on MID() should be the string length, not the char position

[start]:

FIND([Description],"Target name: ")+13

[end]:

FIND([Description]," ",[start])

And finally what you need:

MID([Description],[start]+1,[end]-[start]-1)

This should do. If you want to pursue the 5th and 6th " " approach, I would recommend you to find each of the " " until the 6th.

[1st]:

FIND([Description], " ")

[2nd]:

FIND([Description], " ",[1st] + 1)

And so on. Then:

MID([Description],[5th]+1,[6th]-[5th]-1)