Change the '::' separator to '-', ie:
ppm install Spreadsheet-WriteExcel
If you're running from the ppm prompt, you can search then install by the number of the result.
What an awkward sentence that was! I'll just paste a screenshot:
ppm> search WriteExcel
Searching in Active Repositories
1. Spreadsheet-WriteEx~ [0.41] (none)
2. Spreadsheet-WriteEx~ [0.42] Write to a cross platform Excel binar
+y file
3. Spreadsheet-WriteEx~ [0.42] Write to a cross platform Excel binar
+y file
4. Spreadsheet-WriteEx~ [0.09] Convert a database table to an Excel
+spreadsh~
5. Spreadsheet-WriteEx~ [1.01] Create Excel Spreadsheet from XML
6. Spreadsheet-WriteEx~ [1.02] Create Excel Spreadsheet from XML
7. Spreadsheet-WriteEx~ [1.02] Create Excel Spreadsheet from XML
8. Spreadsheet-WriteEx~ [0.03] A simple single-sheet Excel document
ppm> install 6