Matlab - la funcion xlsread no encuentra el archivo de excel

>> help xlsread

 XLSREAD read Excel spreadsheet (XLS) file.

    A = XLSREAD('FILENAME') returns in A, numeric data from the first sheet in

    the Excel file FILENAME.

    [A, B] = XLSREAD('FILENAME') returns numeric data, stored in A, and text

    data, stored in B.

    [...] = XLSREAD('FILENAME','SHEETNAME') is the same as above but uses the

    sheet named SHEETNAME.  XLSREAD throws an error if SHEETNAME does not exist.

    Empty cells or cells with text result in NaN elements in the returned numeric

    data.

    Leading rows and columns of text do not create NaN elements in the numeric

    result.  This makes it easier to load files that look like this:

    Time   Temp

     12      98

     13      99

     14      97

    City    Time   Temp

    Dallas   12      98

    Tulsa    13      99

    Boise    14      97

    To get available sheet names from file, call XLSFINO with two outputs.

    For example:

    [status, sheetNames] = xlsfinfo(FILENAME)

    When reading date fields from Excel files, the date numbers need

    conversion to a MATLAB date.  For example, if your Excel file contains:

    4/12/99

    4/13/99

    4/14/99

    Convert the data to MATLAB dates like this:

    excelDates = xlsread(FILENAME)

    matlabDates = datenum('30-Dec-1899') + excelDates

    See also LOAD, FILEFORMATS, XLSFINFO, DATENUM, DATEVEC

>> dir

.           Libro1.xls

..

>> xlsread('Libro1.xls')

ans =

    12

    12

    13

    23

     4

     5

     6

    76