In your code there is no any need 1) to introduce new variable 't' of type string; 2) to declare it const. I did that because I set value of the string explicitly and only once.
As for your code you could write simply
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cout << "Enter a line of text" << endl;
cout << "What word do you want to find" << endl;
cin >> find;
size_t numberOfInstances = 0;
string::size_type pos = 0;
while((pos = enterText.find(find, pos)) != string::npos)
pos += find.size();
size_t is a standard typedef name as defined as
typedef unsigned int size_t;
It is simply to write size_t than unsigned int.
As for the loop you start seaching the string with pos 0 in the string. If target word is found you move the position in the string right after the found word that to search an other occurence of the word.
I would like also to note that it is a bad idea to name string as find because there are other entities in C++ that have name find as for example standard algorithm std::find.