A miner is already available with the wallet.
Start the wallet and click -> [ Mining ] from the menu bar, then on [ Solo Mining on ( 1GB required ) ]

If you want to check your hashrate, Click on [ Help ] -> [ Debug Window ] -> Click on [ open ] button below debug log file and check for these line in notepad of debug log.

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2016-02-15 08:43:25 HOdlcoinMiner:
2016-02-15 08:43:25 search finished - best hash 
  hash: 005d75eb8df4c77db77612f87074811879941ff7cf29938242d99d15ed977f24 collisions:996 gethash:005d75eb8df4c77db77612f87074811879941ff7cf29938242d99d15ed977f24 ba:43786 bb:2977807736 nonce:3575
target: 000004c77c000000000000000000000000000000000000000000000000000000
2016-02-15 08:43:25 Hashes Per Second=29 (total seconds=69 hashes=2001)
2016-02-15 08:43:25 CreateNewBlock(): total size 1000
2016-02-15 08:43:25 CreateNewBlock(): nfees 0
2016-02-15 08:43:25 Running HOdlcoinMiner with 1 transactions in block (189 bytes)
2016-02-15 08:44:00 HOdlcoinMiner:
2016-02-15 08:44:00 search finished - best hash 
  hash: 000d27fffc05f4cc792b3c49977106591656292f64274ae80dace0a3443c653c collisions:1021 gethash:000d27fffc05f4cc792b3c49977106591656292f64274ae80dace0a3443c653c ba:217511 bb:3087973704 nonce:4414
target: 000004cdf9000000000000000000000000000000000000000000000000000000
2016-02-15 08:44:00 Hashes Per Second=29 (total seconds=35 hashes=1021)
"