We were recently having issues with MSM in ExpressionEngine, and we were told that we had the wrong version of MSM for our version of ExpressionEngine. We are running version 2.5.5. We were told to contact ExpressionEngine to get a version of MSM that would work with our build. We were sent MSM 2.1.4, the installation went well, and I am able to click on the Edit Sites button and get to the Edit Sites page. However while on this page, the Create New Site button does not display. I'm not sure what the issue is.
This may seem an obvious question, but have you reached the maximum number of MSM site licences you have purchased with Expression Engine?
Each time you purchase an extra site licence, the MSM package you download from EllisLab contains the file system/expressionengine/libraries/Sites.php.
This PHP file contains three hashes: $num_sites_allowed, $the_sites_allowed and $sites_allowed_num which are referenced by the MSM module - no doubt removing the "Create New Site" button if the wrong Sites.php file is used in the installation.
If you do have enough licenses purchased, then it's just a case of copying Sites.php from your old MSM installation (or downloading a package of the MSM again from you Ellislab account and extracting it manually) and replacing it.