How to look up a cell value of an Excel file?

Assuming you can use .csv instead of .xls(x), there are several tools for manipulating CSV files, for example csvsimple.

Though if you want to reinvent some wheels: you can use ReadCSV to read in the .csv file to a key-value structure, then getValue to fetch those values when needed. The syntax of ReadCSV is:

ReadCSV [*] {<label>} [<name>,<value>] {<file>}

ReadCSV will read the CSV <file>, and take columns <name> and <value> (<name> and <value> are the number of the columns; defaults are <name>=1 and <value>=2) and save them under the <label> for later. If the optional * is used, the file is read as a string.

Once the file is read, you can fetch the saved values using getValue:

getValue <macro> {<name>} {<label>}

getValue will fetch the <value> column for the respective <name> in the CSV file read under <label>, and save that in the <macro>.

In your example (removing header row and column), you'd have:

ReadCSV{mydata}{test.csv}
getValuerdPar{Third Parameter}{mydata}
rdPar % prints 7

Here's the expl3 implementation (it is minimalistic: there's no error checking for wrong input, missing data, etc.):

begin{filecontents*}{test.csv}
Third Parameter  , 7
First Parameter  , 5
Second Parameter , 3
end{filecontents*}

documentclass{article}
usepackage{xparse}

ExplSyntaxOn
% Step 1: reading the file
ior_new:N l__diaa_csv_ior
bool_new:N l__diaa_csv_str_bool
NewDocumentCommand ReadCSV { s m >{ SplitArgument {1} {,} }O{ 1,2 } m }
  {
    IfBooleanTF {#1}
      { bool_set_true:N l__diaa_csv_str_bool }
      { bool_set_false:N l__diaa_csv_str_bool }
    diaa_csv_read:nnnn {#2} #3 {#4}
  }
cs_new_protected:Npn diaa_csv_read:nnnn #1 #2 #3 #4
  {
    prop_new:c { g__diaa_csv_#1_prop }
    ior_open:NnTF l__diaa_csv_ior {#4}
      { __diaa_csv_read:cnn { g__diaa_csv_#1_prop } {#2} {#3} }
      { msg_error:nnn { diaa } { file-not-found } {#4} }
  }
msg_new:nnn { diaa } { file-not-found }
  { File~`#1'~not~found. }
cs_new_protected:Npn __diaa_csv_read:Nnn #1 #2 #3
  {
    bool_if:NTF l__diaa_csv_str_bool
      { ior_str_map_inline:Nn }
      { ior_map_inline:Nn }
        l__diaa_csv_ior
        {
          prop_put:Nxx #1
            { clist_item:nn {##1} {#2} }
            { clist_item:nn {##1} {#3} }
        }
  }
cs_generate_variant:Nn __diaa_csv_read:Nnn { c }
%
% Step 2: getting the values
NewDocumentCommand getValue { m m m }
  { tl_set:Nx #1 { diaa_csv_item:nn {#2} {#3} } }
NewExpandableDocumentCommand CSVItem { m m }
  { diaa_csv_item:nn {#1} {#2} }
cs_new:Npn diaa_csv_item:nn #1 #2
  { prop_item:cn { g__diaa_csv_#2_prop } {#1} }
ExplSyntaxOff

begin{document}

ReadCSV{mydata}{test.csv}

getValuerdPar{Third Parameter}{mydata}
rdPar

edefrdPar{CSVItem{First Parameter}{mydata}}%
rdPar

end{document}