Short Sudoku Puzzle Translation

[lew101]

In this puzzle, there's a shaded box with some text (on the left) pointing to it. What does the text on the left say? Thanks for your help!

 

[nekojita]

Both left and right text pointing to boxes are showing the squares you need to fill in to get some present/enter some contest.
The left one says:

全問挑戦コンテストの解答マス（詳しくはプレセントページをご覧ください)

Basically, there will be some page at the back where you can enter a contest if you fill in the right number(s) from the puzzle. Since it's the "全問挑戦コンテスト", maybe there's one from each question in the magazine or something similar.